I've received postcards from business's where the postcard was fully colored, and another one where it was black and white.
I don't know the costs of the colored ones, but i'm willing to bet that it was worth it to try.
The colored one got my attention and it also seemed personal too.
Needless to say... i responded to the colored ones.
I knew the business owner fairly well and felt that it was personal because he took the time to put my name on the card (in red), and highlighted other areas on the cards with different colors.
If you can customize your postcard where you include your prospect's/customer's name on the card, i think you'll get a really good response from it.