graphicxtras.com > Adding brushes to the Photoshop channels and splitting and merging tutorial
They key thing is that you have the channel panel displayed (found via the window menu). The channel panel for RGB is made up of a red channel, green and blue with a little eye / visibility indicator which can be turned on or off.
The strokes can be applied to the red channel and then applied (the same or differently) to the green channel and to the blue channel. You can then re-combine the end result strokes into RGB. Of course, if you are in LAB mode then you can do the same with lightness and the A and B channels. The strokes work in most of the color modes as well as 8 bit and 16bit modes and beyond. The Photoshop brushes and channels (and image modes) are a really flexible feature of the application. And you don't just have to use the brush tool, you can use the smudge tool instead. You don't have to add them to all the channels either so you can fill the red and green and totally leave the blue alone. The end results also depends on the color of the stroke.
Tutorial by: Andrew Buckle
Works with: Photoshop, Elements
Other tutorials: Tutorial links
Videos: Video tutorials
You can use strokes across channels, you can also combine them with effects. Add stroke to the red and then to the green etc and then use an effect in the red or green etc and in most cases you will find that you can combine the artwork with effects - most filter gallery effects are fine but you may find that some cannot be used with a single color ch. such as the blur gallery. How to resolve this ??
You can use the split command which will split the RGB etc into red, green etc images (all in grayscale). You can then use them with most of the effects such as the blur gallery as well as add more strokes to the artwork. If you wish to use filters that cannot work with grayscale then just go to the image mode and set the image mode to RGB. How to get back to a standard RGB document ?? Just use the merge command found in the panel (window menu). You can then bring back the all the images as red / green / blue - you can also even get creative there as you can bring back the red channel as the green channel, the blue as the red and so on resulting in some truly abstract artwork.
You can use them across red, green etc but you can also apply strokes to the red channel combined with a selection; you can also do the same with the green but with a different selection etc. You can use multiple selections and multiple strokes in different locations to create all kinds of extreme color effects.
You can add into the mix adjustments. Go to the individual red and add a stroke and then the green and add a stroke etc and then go back to the red one and you can then go to the adjustments in the image menu. You will notice that not all are available and you will be able to use invert, threshold, posterize etc It is odd that some of them such as exposure are not available but I guess that is the way they are coded and you will find that if you go to the 32bit channels etc even less adjustments are available. If you wish to use adjustment layers then the only way to do this would be to use the panel and then split the image into grayscale and then set to image mode RGB and then use the adjustment layers with the full range of colors and then merge them all back together