When swapping yellow for the 2nd CC, player can still use yellow by selecting the 2nd CC to be yellow.
All my artwork uses that feature, so when sticking to that way you'd be sure all artwork fits each other.
The only exception so far is me using a few yellow pixels for the handles the cabdriver uses to climb into the cab, since the vehicle already needed both company colors for both stuff, but this I considered a minor detail. I believe it was the 3-part engine (DM3 or something?), possibly only used in _ view. You can look it up if you want
Anyways, if you think yellow should be allowed in sprites, just say

But IMO, not using yellow is what this set differs from the other CC-sets out there. Most of them seem to use yellow just like the original TT sprites
