Food coloring is edible so I'd imagine it would break down relatively quick. Distilled + biocide is a safe bet, but I'd also get an actual wc dye if you wanted to have it colored.
I just dismantled my loop this weekend and expected to find lots of growth. My shitty tygon tubing was completely yellowed so I couldn't see the fluid at all. I was pleasantly surprised to find the water inside still clear and free of any growth whatsoever. The tube was just junk. I didn't rebuild the loop, I'm going to wait until a completely new build, and just went with an H105 for now. My loop was distilled + biocide + silver kill coil.