How to paint jeans?

if you want to get crazy, first paint the pants grey and let that dry

brush blue on it, and before the paint dries use a toothbrush to stroke the entire area in one direction, then stroke perpendicular to that (cross pattern)

some of the blue paint is removed, leaving a bunch of lighter colored streaks

keep going until you're done

I've never had to do that though, usually a dark base, blue, lighter blue, and a dusting of light grey for worn spots gives a good enough result

EDIT: You know what, I think you could get a cool denim effect simply by pressing a wrinkly piece of denim onto the blue paint before it dries. Same idea, and probably easier.

Punstarr is right too, the pants need texture for the drybrushing to look good, if the pants are smooth try what I just described.

Now that the Doc mentions it, grey would probably be better as the base coat than black.

You want the bottom layer to be black with your blue dry brushed on top of that for a nice jeans effect. Jeans are also better with texture and folds if you can add any scuplting to the figure. Check out my Jubilee custom from way back to see what I mean for her shorts. Same idea.