All right, here's a quick and dirty rundown of my starfield technique (no pictures). I'm assuming this is in GIMP.
1. Start with an all black image.
2. Rename your background layer "small stars"
3. Go to Filters>Noise>HSV Noise. Make a noisefield with the following values: Holdness 4, Hue 0, Saturation 0, Value 200.
4. Go to Colors>Brightness-Contrast. Increase the contrast by 50. Leave the brightness alone.
5. Make a new all black layer named "bright stars".
6. Make another HSV Noise field with the same values as the first.
7. Go to Colors>Brightness-Contrast. Increase the contrast by 100 this time.
8. Scale up the bright stars layer to double its original size.
9. Go back to the small stars layer and select everything by using the magic wand with a threshold of 255.
10. Go back to the big stars layer.
11. Go to Layers>Crop to selection. Now the excess big stars layer is deleted.
12. Now we're going to give the brighter stars a little color. Make two new layers. Name one "reds" and one "blues".
13. Paint both all black and make HSV noisefields in them as well. Same ol' values.
14. Gaussian Blur both layers with a radius of 5 pixels.
15. Brightness-Contrast both layers. Set Brightness and Contrast both to 100.
16. Add layer masks to both layers. Initialize to "Grayscale copy of layer"
18. Get out the bucket fill with threshold set to 255. Paint the "Reds" layer all red (RGB 255, 0, 0 or HSV 0, 100, 100). Paint the "Blues" layer all light cyan (RGB 0, 255, 255 or HSV 180, 100, 100).
19. You should now see your bright stars on a mottled red and cyan background. This is not what we want. So, set the modes of both the color layers to "Color".
20. Now some of your bright stars are blue and some are red. This is looking good, so merge both color layers down into the "bright stars" layer.
21. Set the "bright stars" layer to "screen" mode so that the smaller stars are visible.
22. Hit "flatten image".
23. There you go, a decent-looking starfield!
24. With practice, you may be able to modify this technique for even greater realism, but always remember the following:
HSV noise is your friend.
The Brightness-Contrast tool will make your noise look more like stars and less like TV static.
To color your stars, make separate red and blue layers. This will color some stars, but where the layers interfere with each other you'll still get plenty of white stars.
Also note: the starfield that other tutorial teaches you to make may look cool, you would never see something like that outside a dense star cluster. A realistic starfield should look like the night sky, but with no light pollution or twinkling. However, you could consider adding a "milky way" band, although I'm not sure the best technique.
