Hello and welcome NightSister! Looks like you're well on track with your Ventress build I must say that the lightsabers you are having printed look amazing!!
Most of the time my husband will airbrush my makeup on for me. The advantage is that you get very even coverage of the paint, and if you're using alcohol-based paints like I do, an airbrush works best. However, I have also seen great paint applications done with sponge or brush, so it's really up to you and what you find is the easiest way to achieve the best results.
As for the fabric, the 501st standard states "The floor length skirt is made of a crushed velvet or chenille fabric". I would use the same fabric for the shirt and the skirt if possible - use the fuzzy side for the skirt to show the texture, and then use the smooth (back) side of the same fabric for the shirt. That way they are the same colour.
Good luck and let us know how it goes!