Hi Ryan,
One of he projects that has been on my backburner for a while is to develop a chromogenic agar medium from edible ingredients - so we can teach some "biosafety level 0" microbiology demos. The end goal would be to be able to distinguish between colonies of safe microbes such as baker's yeast, lactobacillus from yoghurt, and maybe some kombucha isolates, based on color changes on an agar plate.
Promising chromogenic ingredients include plant pigments that change color with pH such as turmeric and cabbage juice (yeast and kumbucha strains will create a lower pH, so you'll get a colored halo around each colony on the plate). But pretty much any brightly colored foods are worth trying: tomato juice, spinach, saffron, you name it. Make an agar plate with each of them, streak with yeast, lactobacillus and kombucha, and check for differentially colored colonies the next day...
I'm signed up to do a 15 minute lightning talk on DIYbio at Science Hack Day at 11:05, and this is one of the three project ideas I'll be talking about (the other two being
Heart in a Jar and
steam distillation of essential oils). I've actually been meaning to make a little international DIYbio competition out of this: offer a cash prize for whomever in the DIYbio community comes up with the best, fully documented recipe using all food-safe ingredients that can distinguish between at least two or more food-safe microbes...
By the way, here's an example of a commercial chromogenic agar plate, used to distinguish E. coli from other coliform bacteria in contaminated water.
E. coli colonies will be purple/blue and coliform colonies will be pink/red:

Patrik