Use layer groups, each group having:
- your grayscale channel image at the bottom
- a color filter at the top.
Set the color filter layers to "Multiliply" mode and set the two top groups to "Addition" mode. Something like this:
Attachment:
Monoceros.png [ 282.66 KiB | Viewed 2133 times ]
You can change the opacity of layer masks to decrease saturation, and the opacity of each group. You can of course use any other color for the color masks.
If the initial images are grayscale, the image mode could be in grayscale which prevent adding colors. Chnage to RGB with Image>Mode>RGB.