How do you color a already colored crankbait without a airbrush.

Sharpies to add black contrast. You can also use a q tip and some dipping dye. 

Rattle cans and some painters tape.

 

Allen

It depends on what colors you're talking about, but you can use Sharpies in their various colors on a bait and it will be semi-permanent.  It will last for a good while but eventually will erode off the bait to some extent.  There are other solvent based marking pens similar to Sharpies which you can get at craft stores.  One I got at Michaels is colored "Goldenrod" and it will make a chrome or silver crankbait into a gold colored crankbait.