The individual methods that Sam used in the previous post are very important to know both separately, and in the combination that he used. For example, his method can be easily modified to superimpose the colors from an actual picture (eg, a flag, not just big, featureless blocks of color) onto stars (...or snow flakes or water droplets, or whatever...).
So, for the example below, I wanted to start with an open source photo of stars taken by the Hubble:
...and superimpose the colors of the American flag:
I did this by using the following very simple layer structure that is exactly analogous to what Sam used, ie, the "color control layer" (the flag) clipped to the areas selected in the starting image:
About the only difference from Sam's approach was that since I wanted more vivid colors, I used the "Hard light" layer blending mode (instead of "Overlay"), and set all opacities to 100% (instead of Sam's 28%)
The result is:
HTH,
Tom M