We have two ways to show which audio tracks get to a given bus. One is the track header color option "follow bus" and the other is alt-clicking in the bus number widget, which then highlights all the sending tracks. And there is vice versa alt-clicking a track to show all receiving buses.
 
A useful extension might be to highlight a midi track according to any receiving synth track that gets highlighted directly or by one if its receiving buses. A small issue is that it may not be desirable to select midi tracks when only audio tracks are desired. Maybe that's what alt-shift or alt-ctrl click is good for. Also, for synths with multiple outputs, there is no internal indicator that a given midi track triggers a given synth track, so the software might have to light up all tracks to the given synth, not knowing which ones really contribute.
 
Might there be a use for coloring a midi track according to a receiving synth track? A complication is that midi is directed to a synth, not a track, and a synth can have more than one synth track. Maybe synths in the rack should have colors too, with "follow synth" an option in a midi track header color. For synths with only a single synth track, a connected midi track could follow any bus receiving from that synth. When a synth has multiple synth tracks that have different colors, someone might have to think of a reasonable default or do some manual coloring.