I'm brand new to Motion and overall it's just a pleasure to use. But one thing I miss from Shake is direct access to the color channels. Is there something fundamental I'm missing or can you not do things like apply a filter to a specific color channel?
Duplicate the layer, use the channel mixer filter (might not be the right name, going by memory here) to remap all channels to your chosen channel, then apply the filter you want to that duplicate - I think that should work....
Freelance Producer/Editor/Motion Graphics Artist
Apple-certified Master Trainer
Author, Motion 4 from Peachpit Press http://www.applemotion.net