I've experimented with darkening down liquidised bread. Using regular food colourings with groundbait seems to be basically pointless, I've tried a few different brands and both liquid and powder. The colour washes out from it really quickly once in the water.
I tried a proper bait dye, Sensas Tracix, after seeing something on YouTube about it. It was better, but to be really effective it needs to be mixed with water and then mixed in. For me, that changed the consistency of my liquidised bread so it didn't cloud up in the same way. (first attempt, it was like dough). I tried putting it through a riddle, using a spray bottle, all sorts. Never got it the same, although I should note that my way to use liquidised bread is to use it with as little water as possible, applied with a spray bottle, ideally no water at all.
One piece of advice...using the Sensas dyes...take and use some rubber gloves!