Hmm well for the most part I just throw a bunch of colors together until it looks good so I really don’t have a set way to make galaxy patterned stuff but I can give you a general way that I usually do to make anything galaxy patterned
First I start with a medium dark color. Make sure it’s dark enough to have some lighter colors that will blend in with it (at least two I’d say) But also light enough to have darker colors that blend in with it (again I’d say at least two)
Next I start with the color that’s a shade darker than the base color. I usually start it in some corner or spot like I have here. I usually make it so it doesn’t spread over the whole thing and leave it in the corner/wherever I have it. I also put the next shade darker in this one as well after I put the first dark shade down. In this case it’s black. I just dotted the black around in where I have the dark dark blue.
Next I add a lighter color in another spot away from the darker area. Then I just dotted in a lighter color than that. Also if you’re not sure how I’m adding in the colors I’m doing it so I’m picking up the pen and dotting it in. So basically just scribble while moving the pen away from the screen. I think it looks better that way but just scribbling on the screen will work too, just don’t make it look too neat.
This step I just started moving the colors into each other. So extend the areas of color so that they’re running into each other and that they’re not in their separate areas.
I think I just added a dark blueish purple in this step. You could just extend the colors you already have into the spot where I added the purple, but I think adding another similar color to the one you have makes it look nicer.
Now I add the stars. I use white and also a more greyish white to make some look brighter than the others.
To make some of the stars shine I add a darker color to the brightest ones (the ones I made white and not grey) to make them seem shinier. I usually use grey for this but really any color that is darker than the white and lighter than the background could work. I think I tried with a light blue and looked ok but grey usually looks the best. Just play around with some light colors and see what looks best. Oh also I added another light blue (well more of a turquoise teal greenish(??) kind of color) because I thought it would look better.
lastly just look and see if it looks done. If you can’t think of anything else to add to it and it looks nice you’re probably done.
If you want I have the colors I used mapped out and everything on another picture if you think that will help, but basically all I do for colors is start with the base, light, and dark colors and go from there. Just add colors as you see fit. It’s easy to change colors on the AC system since you can change the color and have it change on the easel as well so it’s easy to change colors as needed.