Jacquard Textile Colors. I have used this paint to paint spandex and it remains flexible/stretchy and is waterproof (if you mix in a setting medium also available to purchase). You can thin with water, mix colors, or just paint straight. I handpainted 35 unitards with this and was VERY happy with the results. Not affiliated, yadda yadda, just a satisfied customer. I purchased from Dharma Trading.
I have not used it on stretch velvet, my suggestion with that is to be careful of the nap when brushing paint on. When painting mesh, you want something underneath to catch excess paint - paper towel, scrap cloth, wax paper...