Comentario oficial
Okay, a bit of confusion -- it's not the TRACK colour that applies to the display, it's the DRIVE colour. Take a look...
In rekordbox (Export mode), click on the USB drive name, and on the right side, change the preferences for the Background Color:
Here's a photo of that drive (and another drive I set to red) in the XDJ-RX with a song loaded from each: