What is the best way to paint fondant w/out investing in airbrush equipment? I have never tried it but am making a cake w/ butterflies & can't think of any other way to color them.

You have a couple options. You can do the brush embroidery method (my hummingbird cake) or use your luster dusts straight with a brush or diluted with vodka.

I've painted on fondant with lustre dust diluted by vodka. I also use gel icing colours slightly diluted with vodka. I tried painting with just straight gel colour, but I found them a bit gummy and hard to spread around. Once I diluted with a few drops of vodka, it worked better. The colour was just as vibrant but much easier to paint with.