Update...
I've exported my custom colour scheme from X2 and imported it into X3, and the background clip colours have all changed to the pale yellow! So it must be possible, I must have just missed the correct 'name' for it.
Sa far as I can tell though, the foreground colour changes to match whichever custom colour you choose for the track pane in the TV (and the strip colour in the CV).
I thought there was a way to switch off the automatic link between foreground colour and track pane colour but now I'm not so sure.
Mike, I'm off to bed now but I'll dig a little deeper into this tomorrow if nobody else comes along to let you know exactly what's possible.