I'm definitely not an expert, and am happy to be proven wrong, but since the colour box has two transformers in it, I'd be surprised if the ground loop was coming from after the colour box (I.e. a ground loop created between the mixing desk and your pedalboard), but to check that you could try using a cable with an integrated ground lift like this one:
https://www.thomann.co.uk/sommer_cable_sghxu.htm
What I'd suggest though is trying one of these:
https://www.thomann.co.uk/harley_benton_groundbox.htm
Its a fairly cheap solution, and could hopefully fix the grounding issue within your board (or whatever the heck it is that's going on!). But if it doesn't work, you wouldn't have wasted that much money on it!