Well since someone opened the LEGO "Pandora's Box"... let me add a few things from a historical and also sideline perspective (I stayed out of the great Bley War of 2004). And forgive me if I wander around on peripheral subjects... I've got too much to say..
For about 1/2 century the ABS (and prior to that in the 1960s... Cellulose Acetate) were produced for TLG by German chemical giant Bayer GmbH (the aspirin maker). Bayer is one of the worlds largest chemical companies, and had been making Cellulose Acetate LEGO parts, under the brand name "Cellidor" in the 1950s and 1960s. And when the switch to a more stable plastic was introduced in 1962-63, Bayer was the producer of the ABS plastic that replaced Cellidor.
From 1963-73 the red and yellow ABS plastic was a bit of a darker color than it was after 1973. This was due to the colorization process by Bayer included Cadmium... a heavy metal. Red and yellow appeared to be more difficult to color... so the addition of Cadmium eased the process. However by the early 1970s... envoronmentalists and TLG themselves decided that having LEGO parts in land fills over long periods of time (centuries?) would not be good for the envoriment, and therefore removed most Cadmium from the colorization process. (Note: recent testing of modern LEGO elements still show trace levels of Cadmium in some LEGO colors, but the colors would not leach out of the bricks.)
Gray LEGO elements had been used sparingly by TLG from 1953-62... in very few slotted bricks, but mainly in LEGO 10x20 thick baseplates. Then in late 1962 small plates started coming out in gray. This was followed by the large gray baseplate (50x50 studs) in 1964, and other larger gray plates and other (specialty) parts. The use of gray in regular LEGO bricks was introduced in the very rare and highly collectible 1650 Maersk Line Container set of 1974 that had a large number of 2x3 bricks in gray. However it wasn't until the late 1970s (when 32x32 roadplates were introduced, and the 50x50 baseplate was replaced by the 48x48 size)... that LEGO bricks were produced in regular sets (the 722 Basic Set was one of the first), as well as the new Technic (1977) and Space (1979) sets. Also, in Europe LEGO bricks spare parts packs in gray were introduced, but not in USA/Canada.
Then by the mid 1980s the Castle System switched from the (375/6075) Yellow Castle to gray castles sets. So gray became more common in sets... but the gray spare parts packs were still only found in Europe. Now fast forward to 2004. The new gray was introduced. Well you know what hit the fan... and because the number of gray parts had always been greater in Europe than in North America... Europeans as a whole were "more" impacted by the switch to the new gray, although the outrage was on both sides of the Atlantic (and elsewhere).
So there were some very very nasty fights on Lugnet, the main LEGO worldwide LEGO fan site back then. If anything... I do believe that it exacerbated the "diaspora" or proliferation of more LEGO fan sites, although that was also caused by other issues on Lugnet.
So the goodwill that TLG was trying to achieve back in 2001 by starting LEGO Direct... the first AFOL related acknowledgement of the fact that there are adult LEGO collectors out there (TLG wasn't the fastest horse in the race on that one)... was sort of damaged in 2004 by many AFOLs who were outraged and felt betrayed by the color change.
Now... the color changes happened to brown, dark gray and gray... but only gray had been made for many decades... brown and dark gray were of more recent vintage (late 1990s) and were not as impactful as gray was on the general AFOL community. And what made it worse was the fact that LEGO Town and other systems were nowhere as impacted as were the LEGO Castle, LEGO Space and Technic Systems. So there was a lot of arguing back and forth among the LEGO community over how big of a deal the change was.
So Europeans were more impacted than North Americans (no gray parts packs sold here), and LEGO Town and other systems were not as impacted as were Space, Castle and Technic. Ugly ugly battles were fought in the online community in 2004 (and beyond). And like I said... this was almost like the "Tower of Babel" of LEGO... people started going off and starting other LEGO related fan sites as well.
Now go to 2006. What happened around that time was TLG was loosing money in the mid 2000-2010 era. One way to cut costs was to move production of LEGO to cheaper sites... such as Mexico and the Czech Republic (China came later for Minifigs). Another way to save money was to stop buying colorized LEGO pellets from Bayer... which TLG had done for over 1/2 century. The best way for TLG to do that was to stop the storage of vast increases in the number of LEGO colors (remember by 1990 there were only about 8 LEGO colors, by 2006 there were over 40). So TLG stopped the importation of precolored LEGO pellets from Bayer, and just bought the milky clearish-white pellets from multiple sources and started handling the colorization process themselves at production time.
So in doing their own coloring of the parts... using uncolored LEGO pellets from multiple sources, and adding color at multiple sites, with slight variants (temperature, quantity, etc.) the quality of "even-ness" of coloring of parts began to suffer. One of the first big sets that saw this problem was the color purple in the first Harry Potter Knight Bus. The colors became a problem... but TLG greatly improved their "replacement parts" service to compensate for this. And that is basically where we are today with the problems of variances in LEGO colors... it is a problem... and TLG will provide replacement parts when so requested.
Well I've been long winded enough... but I think I covered the major issues...
Oh... one last thing... trans-colored parts are not made of ABS plastic (although they were made of Cellulose Acetate prior to the mid 1960s)... ABS is not trans-clear... so Polycarbonate is used to produce those colors.
P.S. I've pretty much gave away 1/2 chapters worth of information on the next version of my LEGO DVD... only available as a desktop download for future issues... with many new chapters... which will be out in about in 6 months and will be FREE to current DVD/download owners. I've been in weekly contact with the TLG Archives/Collections folks... and they have been finding many rare and interesting items never seen before... because I now finally have scans of old retailer/customer catalogs going back to DAY 1 of LEGO... 1949.... and been asking them to look for something.. and then we discuss what it is, where it was produced, and how/when it was used (a symbiotic relationship!
). Many new items in the next version, some never released.... sorry for the shameless plug!
Edited by LEGO Historian, 07 March 2013 - 08:44 AM.