What does this coloring mean?

The orange color means you have filled 75% of your storage for that resource (so you should be ready to spend them, otherwise you'll be wasting time and/or food). Kittens are considered resource by the game so it will always be orange colored

catpower and science have different color but they do not have any real meaning, they're only there to help you differentiate themselves from other resources. Later on you'll find a ridiculous resource when hunting, that will have orange color AND glow

(Partial answer, so not complete (for now) and still looking for the answer what the coloring of the resource means).

The orange resource amount is just (very close to) the maximum, I just discovered that. Furthermore, there also seems to be a wider range where the resource amount has a different color then the orange, it's sort of salmon-colored. My amount of kittens was then 80% of the maximum. At 90% (9 out of 10 kittens) it was still salmon colored.

So orange resource amount means that it hit (or is very very close to) the maximum. Salmon colored resource amount means close to the maximum.