A small amount of time, over the last few days, has been spent on a change of paint type and application method... a bit of proper airbrush work - not a computer in sight, at least this time. Rather than oil or watercolour paint, and for this job, it really is the realm of Designers Gouache, in this case, from Winsor & Newton.
The airbrush I use, and have done since college, is the DeVilbiss Aerograph Super 63 and really is a superb instrument. And no you don't need three of them unless working as a professional illustrator or something where they are being used all day every day which these were at one time.
The only thing missing really was a piece of line board used for most technical ittustration... maybe next time.