Grayscale printing on the back is included, but I'm wondering if it's worth the extra cost that the printer charges to have the address side printed in color. It's not that it's terribly expensive, only $30 for 1000 postcards, but no sense throwing away money if it doesn't make a difference in the response rate.
Extra cost? Most printers hardly charge anything when going from a 4:1 to 4:4 postcard. My printer charges a whoppin' $1.00 premium to go 4 color both sides.