>>29706189It's less that the colored plastic is more expensive than the regular plastic and more that having a cartridge with a unique color of plastic means you need to do a production run of cartridges specifically for that color.
If everything uses the same basic grey, then you can just mass produce that grey shell in huge batches and use it for everything. If you want a game to have a specific color, then you need to do another much smaller production run of that color, and that eats into the economy of scale bad.