It is risky. Color ink and/or colored paper will reduce the contrast between the bars and spaces. Also remember that many scanners use red light. If you print the bars using any shade of red, the same amount of light will reflect off the red bars as the white spaces. Also, printing black bars on a colored paper will also reduce the light reflecting off the spaces and reduce the contrast. Other colored inks will also reduce the contrast ratio between the bars and spaces and greatly increase the probability of an unreadable barcode. If the black bars and white spaces are too glossy, the symbol also may not read. A real no-no is printing black bars on a silver can. Many large chains now fine or disqualify vendors who supply products with bar codes that do not scan. If you print the UPC symbol with color ink, you run the risk that the symbol will not scan. That could result in you losing a big customer.