Add new color to critical threat rolls that aren't 20s

I have a couple of characters that are use weapons with critical threat range of 15-20 and 18-20. Currently, anytime a critical threat is rolled, the die roll in chat appears green. How can I change it to a different color (dark yellow for example) when the result is a critical threat but not a natural 20?

The only other color available is blue, for a roll containing both a CS and a CF. [[1d20cs>15cf15cf16cf17cf18cf19cf1]] This will give you a green on 20, blue on 19-15, and red on 1. There might be some API trickery to do something similar, not sure. The above does not seem to work properly with a /r roll, for some reason. Maybe the blue never shows up at all with those rolls? Not sure, I never really use them.