I sometimes do it....either airpbush or spray paint of the "base" color. Good way to get a solid overall color to quickly get a squad painted.
The only problem is the colors themselves aren't as easy to determine highlights and shadows as the (for example) Citadel Colors...
The best way to do this is if you're mass painting.....I'll spray like 20 Ducal Militia charcoal gray, then drybrush them all. After that, you can add details and spot colors. At this point, they are perfectly table worthy....and it's up to you how much work you do on insignia, washes, etc.