This should of been ask in the hydro section. but here goes.

Any rocks will not due. The medium must be able to hold some water. It's a caition exchange thing. I.E. hydrotron expanded clay pellets. Even lava rocks for a gas grill, bust them up good with a hammer and wash. Rockwool comes in all sizes to do the job. Cocofiber, perilite. Rocks are best I think.

Yes fill the water level just under the netpot. The trick is air stones. The more air you pump in the better. The bubbles splash the netpots. Bubbler or DWC style.

No food coloring will not and I will tell you. Many of the nutrients used will stain nice white roots but to no efect to the green. Mainly genetics.

I went to the hydro store and did see a product that claimed to even turn tomatoes purple. I told the guy that's like false advertising. LOL