PPro CS4: Mixing colour with grayscale parts of a video

Question

Hi Guys
I have 1 video track on PPro that was shot in full colour. I just want to add an effect of converting the whole video into grayscale (or black & white) but keeping a red colour "layers" on the same video.
I have seen some of the ads on TV use this effect & I was wondering how can I do the same thing. For instance I saw a Kellog's rice krispies ad, where the people and the background will be black & white but the Kellog''s box remains blue throughout the whole ad.
I was thinking maybe I should export frames one by one from PPro to Photoshop and edit the individual frames from there using the selection tool like quick selection tool to separate the black & white layers from the colour layers and then export out of Photoshop the finished video back to PPro: but then I thought this will be time consuming and not necessarily the best of doing it.
Any help will be greatly appreciated.
Many thanks
WalT

Answer 1

Copy the clip to the next higher track and apply a garbage matte to this track to limit the area where the color pass is applied. Both tracks have a color pass filter. In the example below I have blue on track one, as in the previous example. I copied this to the next higher track, applied a 16 point garbage matte to one of the pipes and applied a red color pass filter. The result is two different colors are passed.
You use this technique to modify the color a bit for each pass. Keep the similarity low on track 1 and increase it on track 2 with the limited matte. Around the hat for instance, but excluding the lips.