I thought it might have been done with an airbrush. I'm an illustrator, and for many years I spent 10 to 12 hours a day with an airbrush in my hand (go to
http://www.dawsondoes.com/illustration_portfolios/traditional_illustrations.html to see some of my older stuff). Nowadays I do it all on a Mac. From time to time I miss using brushes and airbrushes; then I remember what a pain they were (paint over-spray all over everything, spilled paint, etc.). Your stuff looks good. Keep at it -- it's a dying art.