Karl Soule has followed up his look at color space processing in Premiere and the Premiere Pro Effects panel with What is YUV? Here's some flavor:
'When effects are used on a video frame, sometimes the effect needs to convert the values back to RGB before the effect can be applied. In 8-bit or 16-bit-per-channel color space, there can potentially be "rounding errors" when performing this calculation. This can mean situations where pixels that should pass through an effect unchanged will, in fact change in an unwanted way.
Effects in Premiere Pro that have the YUV logo do the processing directly on the YUV values without converting them to RGB first. At no point are the pixel values converted to RGB, and you won't see any unwanted color shifting.
32-bit color space has the color precision to convert cleanly from YUV to RGB, and will not cause any of these rounding errors. In Premiere Pro, all of the 32-bit effects are "safe" to use.'
Update: Karl continues his series with Color Subsampling, or What is 4:4:4 or 4:2:2?? See also DV Pix- Sampling Methods by Adam Wilt and Chroma Sampling: An Investigation by Graeme Nattress.
Update 2: Karl adds, "...in the engineering world, when discussing YUV color channels properly, there are two separate terms for analog color signals and digital colored signals. The term Y,Pb,Pr is referring to an analog signal, and I used it in a couple of my posts to describe a digital signal. The correct term for the digital signal is Y,Cb,Cr. Graeme Nattress from RED pointed out the mistake in the repost of this blog on Pro Video Coalition."
Update 3: Allan Tépper asks, Can editors and colorists finally scream: “Look Ma’, no professional i/o!”? in Does Premiere CS5 achieve the “impossible dream” for critical evaluation monitoring? at PVC.
Update 4: Adobe explains some stuff under the hood in this March 2011 thread, How does P Pro handle YUV (YCbCr) and RGB color spaces?