I usually suggest have 2 separate item codes - one for unpainted and one for painted.  You bring in the unpainted, then created either a Kit or a Bill that contains a line for unpainted items and a misc charge line for the cost of the paint.

You use the same misc charge on the PO to the Painter. 

...if you want to have the painted items sit in inventory (built using BoM production), having the 2nd painted item as Lot valuation would allow you to use color's as Lot #'s to easily see quantities of each.