Haha! I found the answer!
...now if only I could get this stuff in the UK!
http://www.thesage.com/catalog/Colors.html#GrapeColor"Color only turns purple in an alkaline environment. Color is blue in lotions or melt & pour soap base. To mix a large quantity, add 1/2 tsp color to1 oz dropper bottle, fill with water, insert dropper tip and place over cap on bottle. Shake well."
Fruity essential oils, especially lemon and citrus ones, are more likely to be alkaline. So is rosemary and thyme. Also almonds are alkalising (I put a few drops of sweet almond oil in many things anyway) along with olive oil, flax oil, hemp oil, avocado oils, evening primrose oil and coconut oil. I expect if you put the dye on the outside, and the alkalising ingredients in the middle of your bath bomb, then it might change the colour of the water around it as it fizzes down. Naturally, it would blend in after a while, and what you would could probably end up with is more ph neutral bath water, once it all dissapates (if you were clever!). Now I need to work out how to get me some of this stuff, some ph papers and test it out! Think I will be talking nicely to my family in the US!