Why do some sets have such weird brick color choices hidden inside?

[Cody Startale]

Now, I've been wondering this for a while now, usually when assembling a new set and just shrugged it off eventually. But now, I am really curious, so I apologize if this is common knowledge that just escaped me so far. But I would really like to know why sometimes, TLG goes for outright weird or almost nonsensical color choices for their sets. Specifically parts that will be obscured by other bricks eventually! Like with MOCs, people use to build the "inner" part in whatever color they have available and have the "exterior" colormatch and represent what it's supposed to look like, it's a common practice. So I thought it might be the same for TLG? But when I put together the new Mos Eisley Cantina yesterday, I was just outright confused by this weird design choice:

As you can see, you need to add four 1x1 round plates in the first step... only to have them be completely covered by the following steps! You never get to see those pieces again in this build! Now, I would get why for the mentioned reasons, they would just go ahead and let you build like four trans-red pieces into it, because they happen to be around in tons anyway for the stud shooters. But why do they make you even use two different colors?? One of them being silver? I imagine those are somewhat more expensive to produce than regular colors? Also, they give you extras in the set anyway, so it's not like you would be short on trans-red studs, for example. I just don't get it. Is this a common practice for TLG because they over-produced certain colors in the year prior or something? Or is this like some weird inside joke that I don't get (excuse the pun)? I just found it so weird to put this together and pay attention to grab the correctly colored studs because I didn't need 4x trans-red, but instead 2x trans-read and 2x silver, only to have them be placed inside the build and then immediately put something else on top of them and have them never be seen again by anyone.

Does anyone know the definitive reason why this practice is in place sometimes?

[Robert8]

It's to make easier the building process for younger builders

Those are made in different color, so they'll stand out. It would harder to build a set, if all the pieces (or most of them) are black

That happened a lot in the Monster Fighters theme, where most of the builds were pretty dark-colored. There were yellow, blue and red pieces included so they'll stand out during the building process

[NathanR]

I can think of a few reasons for multi-coloured bricks in the interior. Sometimes it's done to help highlight which way round the model should go (for example, to show where the almost identical side-panels of BB-8 attach), sometimes it's to highlight the attachment points in complex builds, like when you put the sides on the Super Star Destroyer.  Sometimes the random colours are just to make sure that each piece appears in a single colour throughout a given set.  

This particular case is an odd one.  Were all the 1x1 round plates included in a single sealed bag? If so, the colour change might have been done to ensure a more obvious weight change for the bag during the packing process.

[Johnny1360]

As has been said, I believe it is often used to properly orient different sub assemblies, avoid confusion. Whatever the reason I am grateful as if gives me a greater variety of pieces for MOCs.

[JopieK]

What @Robert8 said. This is how at least LEGO Designers explain it. The amount of colors is one of the factors they take into account when targeting a certain age. More colors make it easier to read the instructions and thus are targeting younger agegroups.  

[Littleworlds]

The choice of pieces in this one is quite interesting though. Usually it is standard colours, like red, yellow and blue. I haven't seen translucent and metallic used much as purely structural pieces. Metallic actually never before. Maybe the choice is also influenced by the production and logistics.

[RetroInferno]

It's also to keep the build interesting visually and fun: I'm always happy getting some bright colors to fill up some sets even if they remain hidden in the final product; especially if you are building big grey blocks like some of the star wars sets. 

[Grrr]

The colour swapping as several mentioned above is to help orientate the model during the build. 

Typically, the colours used for this sort of thing are basic colours; however, I believe there is some discretion with this provided they contrast, and sometimes designers will use colours that will work well in alternative builds or mocs.

[icemorons]

I prefer to think of the reason from a more fantasy-esque perspective:   You're installing "the heart of the beast"--  Sometimes it's red, or yellow, or blue, but it could also be gold...     Logistically speaking, I'm sure it's to make reference easier for complex builds or younger age-groups, as speculated above.

[Phoxtane]

Are those pieces in those colors used elsewhere in the build? Doing so would cut down on the number of unique colors that would need to be packed into a set, reducing that much demand from Lego's constantly-full production lines. It's probably what other have said regarding age groups, etc., though.

[splatman]

Might be TLG's way of clearing out excess inventory of certain parts, so it's likely not all production overruns are ending up your local friendly neighborhood LEGO Store's PAB wall.

[dagupa]

As point out before, this helps to give the model a proper orientation. It usually helps people when there's a rotation in the building process, but it's not limited to that. In this specific model that is exactly the same shape until step 12, those red round plates and the red 1x2 plate help builders to focus the attention back to the model after coming back from "reading" the parts list. People can lose focus of the model when they are not concentrated or the instructions are far from the model or just because they unintentionally moved the model between steeps. Without these differences, builders would loose time wondering if the model is in the correct position.

But, why those color? For all the reasons stated above. Managing inventory and production lines is a complex process: so many combinations and interactions. I even think (and this is my opinion) that TLG uses Heuristics instead of searching for an optimal solution. That means that the exact answer to your questions is only in the head of those who took the decision and that my answer is just a guess :)

[Faefrost]

20 hours ago, splatman said:

Might be TLG's way of clearing out excess inventory of certain parts, so it's likely not all production overruns are ending up your local friendly neighborhood LEGO Store's PAB wall.

This is a little bit of it. We as AFOL’s tend to think of sets by “10 cents per piece” or something similar. But that’s not how designers view it. They are given a part budget to work with. Each part has a certain specific budget cost. Each color or color change has a cost. Parts that are already surplus and do not require injection molding factory time have a lower budget cost than those requiring molding. Sometimes it also has to do with the packaging lines. They can only have so many unique parts placed in a given bag. Each bagging station only has so many hoppers. So each part or hopper change/add requires budget. Finally that is balanced by the need to keep clear color distinctions in the sets and instructions. Not just for young builders but even for AFOLs. I challenge anyone to build a Megobloks Call of Duty set and not go mad trying to discerne colors. 

[ShaydDeGrai]

I was chatting with a Lego Designer about this a few years back and many of the points he covered have already been raised, but for completion I'll try to channel what _I_ was told:

When it comes to parts that will eventually be buried in the build color choice (and sometimes, though less often, geometry, i.e. using two 1x2s in place of a 1x4) there are several tradeoffs ( in decreasing order of importance) to consider:

1) Visibility, clarity and visual interest - using high contrast parts makes the instructions easier to follow; it makes the model easier to orient if the build up to that point has been highly symmetric; and, it breaks up large seas of monochrome studs with "reference" parts and in general makes thing less bland.

2) Cost and availability - molded parts sitting in bins take up space and represent money that has already been spend but not yet converted into a revenue stream.  Parts that need to be fabricated represent addition expense in money, time and materials.  Where possible, spending down existing inventory is preferable - it feeds the revenue stream while freeing up slots in the warehouse to store other parts.

3a) Production line constraints - polybags are filled one item at a time (though sometimes that "item" is a sealed polybag itself) this means that each type of part (shape & color) requires a dedicated filling station on the production line.  Ideally you want the number of unique parts in a bag to be well aligned to the capacity of the line (number of potential filling stations).  Lines vary in length and you don't want to tie up a high capacity line with half the stations idle just because you went over the capacity of the next smallest line by one part type.

3b) Filling station constraints - filling stations usually try to separate and drop parts one at a time even if the station is supposed to add 400 technic pins to a bag.  Stations may count parts optically or by weight or both.  In the weight case, basically the system weighs the bag coming in, drops X parts that are known to weight Y grams each and turns off when the bag is X*Y grams heavier than it was when it started. And it tries to do this really, really fast. When the system screws up, it usually errs on the side of caution, dropping too many parts rather than too few (which is, in part, why it's more common to see spare parts in a kit than missing pieces).  Scales often have a saddle curve where they measure this extremely accurately in some mid-range and accuracy falls off on either end.  Depending on the filling station, it might be preferable to add "at least B but not more than C parts per bag" to minimize counting errors.

The designer I spoke to was acutely aware of points 1 & 2 and said that when it came to production line issues he'd gotten an overview that end of the process but other than some vague guidelines it wasn't really his problem.  There were people whose job it was to figure that out and he only got involved when someone would call him and say "would it be possible to substitute M for N or use more O in place of P" then they'd refine the design.

So I guess the bottom line is that "random" interior colors are anything but random.  It's driven by everything from shaping the user experience, to long term inventory management, to packaging and quality control.

[kieran]

It's to make the building easier, personaly I hate it, but I remember Lego when flag ship set had a thin instruction booklet 

[sed6]

All of the above reasons are good and make sense. I'll add that I believe it's to add variety to your Lego collection when your sets are disassembled. Imagine if you disassembled a gray Star Wars set or green Emerald Night and all you had was gray and green...boring! 

[bombcar]

Another reason (obvious in the Creator 3-in-1 sets) is that sometimes the pieces are used in alternative models. And sometimes that applies even if they only release instructions for one model. 

And sometimes two sets are designed to work together, and that dictates parts choice, too.