Colored bulbs are one way to go, you can also take the clock apart and use translucent vinyl. You have SO many more color choices using the vinyl. Disassembling the clock is easy too. If you go the vinyl route, stick it to the white diffuser and then make sure the white side faces the bulb, and the colored side faces the car interior. You'll really like the results.
Your local sign maker should have some scraps he'd give you, but he shouldn't charge you more than $1 for a scrap if anything.
Here's mine:
