You can get clear acrylics from tamiya. The available colors are Clear Red, Clear Yellow, Clear Blue, Clear Green. You can mix the ones you are mixing. I will have to say these dry quite a bit faster than normal artist colors and are alcohol based acrylics rather than water based. I would recommend getting the thinner from Tamiya as well though to be honest I only used these in the beginning of my doll interests a few months ago. They will go through MSC as they are alcohol based and stain your vinyl if you're not careful. I prefer soft body Liquitex now for opaque painting.
the colors:
http://www.scalehobbyist.com/catagories ... =a&man=TAMthe thinner:
http://www.scalehobbyist.com/catagories ... =0&man=TAMEDIT: I believe those clear legs and wings are probably airbrushed with hobby color similar to the Tamiya paints. There is also a Mr.Color brand that comes in either lacquer or acrylics. The Mr.Color is a lot harder to find and more expensive. If you don't own an airbrush it will be really hard to get a smooth transition for clear colors since each dab of paint will darken the previous layer. With the fast dry time, you would be hard pressed to paint wet on wet as well so be prepared to get hard edges.
EDIT2: You can also get a similar effect using Pastels. However the effect usually won't be as transparent or as vibrant since you can't layer that much and preserve the transparency.
Ah....I know all this from being obsessed with garage kits hahahah...lotsa clear parts there.