Cakewalk does not really has "Instrument" tracks. That is just visual feature, which represent combined parameters from 2 tracks.
So, while track view shows you "MIDI track" color, Control Surface API reports "Audio track" color. Feel free to report Cakewalk this bug...
In the mean time, you can "Split instrument track", change corresponding audio track color (you will see it is still default) and then combine tracks back into
"Instrument track".