Click here to download the PhotoShop action to combine grayscale channels into RGB color.  Instructions for usage follow.

Figure 1 shows a Color_Combine PhotoShop action, named Colour_compositeEx. This is an extension of the ESA/ESO/NASA Colour_composite action that can be downloaded from their web site.
See URL...
http://www.spacetelescope.org/projects/fits_liberator/download_v1.html

I have simply added an action that automates the process and includes Brightness/Contrast adjustments to each layer. To use the action, first open the layers window with the F7 key. Now make a copy of your grayscale blue image, rename it RGB and leave it as grayscale. Then paste the green grayscale image onto the new RGB image and finally paste the red grayscale image. You now have a grayscale image with two layers and Layer 2 is active... see Figure 2.

Now you are ready to execute the action to do the color combine. Under the Colour_compositeEx action set, start the Color_Combine action. Answer "Don't Merge", if that question comes up. Now you have color for each layer plus four adjustments layers for each... see figure 3. After making any further adjustments, all you have to do is flatten the image and paste it onto your luminance.