YOu can also use clear, but put white tape on the inside of the frame.
If it the taller profile, you can place white matboard (or even a color to match the backing).
Baer mentions the white paint option.
But here's the nifty thing. Spray the backside of the clear. The side that faces the frame. Leave the clear side a clear. The white will look better that way. It won't look painted. It will look like white plastic. More depth.