I have been working more and more with compositing full on 3D adverts, and was quite happy extracting matte runs from so called ID passes with the usual methods that most people have seen online before, for example Channels – Shift Channels effect to move the green channel into the alpha. When you have been given also Cyan, Magenta, and Yellow in the color channel things can get a little more tricky. Some might be tempted to use the keying tools found in AE to extract the colors, other might extract the channels once, and then precomp to extract again to reach one of these other passes. What I have proposed is using the Channel Mixer effect and some simple expressions to allow a slider that lets you flip between the 6 Channels. This workflow also allows Masks to be drawn so you can further adjust the mattes that have been extracted. I spent the time to set this up as an animation preset as it is much faster than building each time the effects needed to extract the channel you want, plus if you decide later to change the channel you want to use you have to adjust your work. With the slider this is a breeze. I found also if you stack various layers above each other you ought to set the transfer modes to Alpha Add this will tighten the alpha channel where there are joins. This is normally due to anti aliasing on the edges
You can download the animation preset from here and also watch a demonstration of how you might deal with color ID passes, starting with ones that just contain red, green, and blue moving onto all 6 colors (the extras being Cyan, Magenta, and Yellow) I finish with the animation preset being used.
I hope this preset is useful to speeding up your work, and becomes something you use frequently I know myself I do.