Hi, I have painted many leotards and the paint I use is Jacquards textile water based fabric paint. It works really well and has every color you can think of, neon, dark, gold, metallic, etc. It also does not crack, and is permanent after ironing. (On occasion I sometimes also use the a cheaper kind from Joann fabrics classed Matte Tulip soft fabric paint. It also works well but is not as good of a quality, and doesn't have a large selection of colors.) If you paint the leotard while it is lying flat, even when the gymnast puts it on, and the fabric stretches there are no gaps or strange looking lines. It really does look just as it did laying flat, so I would suggest trying Jacquards.
Fabric markers do work well, however I have found them to bleed, (even if they were permanent), it doesn't usually show from far away, but on light colors it certainly can be a problem.
Jacquards is also good to try out because if you make a mistake, it is very easy to cover with another color, and have it not even show at all.
And to bluepainter, in regards to your dark blue unitard: If you buy a white paint, and cover the area that you wish to paint with that first, then you can get beautiful vibrant light colors on a darker fabric. I would suggest doing the design in white, and while it is still slightly wet, to use a medium value blue, apply to one end of the white design and slowly blend down to get a gradient effect. I hope that makes sense.
And hope that helps.
If anyone has any more specific questions about fabric painting feel free to ask, I have had quite a bit of experience in the area and would love to help out.
Good luck to all you painters!