Is it legal to have under glow on my car?

In California, any vehicle may be equipped with a lamp or device on the exterior of the vehicle that emits a diffused nonglaring light of not more than 0.05 candela per square inch of area. The light may not display red in the front, but may display other colors. The light may not be installed within 12 inches or in a position to interfere with the visibility or effectiveness of any required lamp, reflector, or other device on the vehicle. This is from California Vehicle Code Section 25400 Lighting Requirements.

Check with your city and county statutes as well as state law to find out if this is legal in your area. The laws differ across the country.