Illegal is a utility Matchbox shader for Autodesk Flame, Flame Assist, Flare and Smoke that highlights any pixels that, when converted from RGB to YUV and back, are above 1.0 or below 0.0. Certain RGB combinations can become illegal when converted to YUV and back to RGB.

Note that this is experimental and not specifically calibrated to any particular gamut standard (like EBU R 103) and might not reveal all illegal colors; it is currently meant to serve as an 'early warning' that potentially out-of-gamut colors may exist when converted to YUV.

You can also switch off the RGB > YUV > RGB conversion to look for over/under values in the original RGB source.

Grab Illegal from the Logik Matchbook site.