I use white gesso for priming, but I use it way watered down and for only small scale stuff. I have tried it for 28mm and personally I was not very pleased with the results. I found that if i didn't water it down I'd get softened details and often crush strokes. If I did I
Water it down , I would get bubbles in the primer. I've heard black works better, but I've never tried and I can't imagine color would make difference. There is a post somewhere with a few shots of larger scale minis primed with gesso on my blog smallscaleminis.wordpress.com, but mostly about my experience with 6mm scale. I've heard good things about vallejo surface primer brushed on, that's the next thing I'm going to try.