New in V0.6: Optimizations for diameter 11 from Mark DeNies. Thanks!
This thread talks about other ways of doing this (with AVISynth) if you're interested.

Smart Smoother IQ is a filter for VirtualDub similar to Donald Graft's Smart Smoother, but operates in IQ (chrominance/"colour") space instead of RGB. Luminance (Y) is left intact. It was designed to reduce the appearance of "rainbows" (cross-colour artifacts) seen in certain anime DVDs, such as AnimEigo's Bubblegum Crisis DVDs and Pioneer's Trigun DVDs.

Diameter, Strength, and Interlaced source footage are provided as IQ smoothing options, similar to the Smart Smoother.

Here is an example of the output in RGB and separated Y, I, and Q views. The rainbows are a little difficult to see in a low-res JPEG, but the noise should be visible in the IQ channels. This is after one pass. Another pass reduces the rainbows further.
Before:

After:

Programmers: Emcee one's filter request site.