Haven't been around in a long time... still expanding and now rewriting my (soon to be 4000 page) digital LEGO guide...
But in the LEGO color discussion... TLG didn't make gray and green bricks for several reasons. One was they were weirdly stingy about producing a lot of different LEGO colors in bricks for decades. When the tube bottom bricks were introduced in 1958, LEGO bricks/plates were only found in red, white, blue, yellow, clear (and starting in 1961 (EU), and 1963 (UK/AUS, USA/Canada)) black bricks.
Also gray plates were introduced starting in EU in late 1962, in USA/Canada in 1963, and UK/Australia in 1964. Green plates were introduced in USA/Canada in 1963, and elsewhere much later.
The first gray bricks were found in the 1974 introduced 1650 Maersk Line Container Ship, and in regular sets (in about a dozen sizes) in the 722 Universal Building Set in 1980, as well as a few in (starting in 1979) Space sets. When the 375/383 first Castle sets of 1978 were introduced, they were not intended to be the beginning of a LEGO System... just a pair of sets in the parts colors that were available. Had these sets been introduced 2 years late (1980), they likely would have been gray. When the actual LEGO Castle System was introduced in 1984... gray became the color of choice. So if TLG was worried about gray being used for war toys by children, that worry was pretty much over by 1980 (of course after the yellow castle introduction).
The reason for using yellow to represent minifigure heads had its' origins in the mid 1960s, when yellow was the best color suited for LEGO built figures, such as those in the 905 Doll Set (USA/Canada only), and the 321 Clown Set (Denmark, Norway and Japan only)...
https://www.bricklink.com/v2/catalog/catalogitem.page?S=905-1#T=S&O={"iconly":0}
https://www.bricklink.com/v2/catalog/catalogitem.page?S=321-1#T=S&O={"iconly":0}
Also, this is unknown to most LEGO collectors, but the introduction of black bricks in the 1960s, made for some "alternate builds" of many of the 316-324 sets of the mid 1960s, as African figures...
TLG was very stingy in creating new colors for some very odd reason. They made dark gray, tan, and Maersk blue bricks for the model shops, but not for general sets until much later. In 1985 I saw a model at a Detroit KMART of a very large model of Brussels Gothic City Hall (Hotel de Ville) using ONLY tan and old dark gray colors. I became angry at the time, because they were being dishonest to kids of that era, of what could be built with LEGO colors that was actually available to the public. And it wasn't until the 1990s that other color bricks started being produced for sets. And since 2000 we started having an absurdly large number of LEGO colors. It's almost as though TLG was atoning for their lack of colors stinginess of the 1960s-1990s.