ProVideo Coalition.com: Adobe Beyond Adobe by Adobe | Vendor Blog.
This post has a very easy to understand explanation on the differences between 4:4:4, 4:2:2, and 4:2:0 color sub-sampling in video.
It also points out that the Canon HD-DSLR cameras shoot in a compressed MPEG-2 format with a color space of 4:2:0, which doesn’t necessarily mean that you get “more color” if you convert the clips to 4:2:2 or 4:4:4.
One of the best reasons I can see for converting or transcoding the H264 encoded Canon clips to something like ProRes422 in software like Grinder, is that the resulting uncompressed file (or more correctly less compressed) file will give you more wiggle room to push pixels around in compositing and/or color grading.