I didn't think about making them a brush instead of a pattern, that would probably be better if you're not covering a large area and especially if you want all your circles to look the same. And it would be a lot easier to change the color. Good eye, mahvin!
GIMPtricks has a great video on how to make a pattern (in this case, scanlines) and making brushes is the same way with just one extra step.
So to make your brush, it's very similar:
1. Make a new image, set the size to be a larger than your circles.
2. On the New Image window, choose Advanced Options and change Fill With to Transparency.
3. With black as the color, place your polka-dots wherever you want, at whatever size you want.
4. When you've got it how you want it, go to Image > Mode > Grayscale . (extra step)5. Go to Edit > Copy .
6. Go to Edit > Paste As > New Brush .
7. Call it whatever you want, both the name and file can be the same.
And that's that! You won't have to refresh, or at least I didn't, the new brush just appeared. You can change the color like you do with any other brush. You can scale it up or down, but it gets a little blurry.
When you're done with the polka-dot brush forever, you can delete it by right-clicking and choosing delete.