Yeah, but if we are not going to use real photo-chemistry (which is not likely to give color OP wants anyway) then we could just use colored water....

I don't think real photo-chemistry change color enough to be effective visual queue, either. I like the food coloring idea. Or you could use water color. Or you could do B&W and paint the color in. It's an art project, not a science project, right?

Red cabbage idea brings back memories.... I did that when I was 6 years old. I remember tasting it too. Yuck!