I have not upgraded to 3.1 yet. I imagine I’ll have to redo my work on icon colors. The stock code allows a configuration file to specify altitudes and hues. However, I’ve expanded that to include the ability to change saturation and lightness as well. My color scheme is intended to be as intuitive as possible. Dark brown is used for planes on the ground. Brown is used because the dirt is brown. I also use lightness to indicate height. As we look up at aircraft, the highest ones are pale blue from the scattering of light from the atmosphere between us and the planes. So my scheme starts with dark brown at the surface, gradually changing to a rich green (think tree color - trees are higher than the ground) for airborne but relatively low aircraft. Higher aircraft are blue and paler. Very high ones get a pink tinge, like the pink you get from dawn or sunset-light high clouds.
Here is a chart of the colors I’ve specified in the config file. The ones in square brackets are not direct entries but are what the interpolated colors are. Because the perceived brightness is not linear with the hue (blue appears darker than green which appears darker than yellow), I had to add several entries to make it all look right.