RTF is a global modification that replaces all towns in the game.

I think I'll do it this way:
1) Base and upgrade in blue, and alt-upgrade in green (variant for jungles or islands).
2) Base and upgrade in yellow, and alt-upgrade in green or red (variant for the desert).

I like both options, if the upgrade will differ from the base in shade. In general, we need to decide where the lizards will live (in the deserts or in the jungles/swamps). After all, the choice depends on the color of their skin :D
However, the helmet and the head itself make them more suitable for the 1st option. Desert lizards would suit the head of some kind of skink (or Australian moloch).

And in any case, the red option is too poisonous in shade. It is better to change it to some kind of crimson, or cherry (as an option). However, then the desert lizards will be too similar to the Cabirs... this is not suitable.

In short, I like the 1st option a little more.