If you are Gimp users you should know the Sparks brush.it is the yellow brush at the bottom of the brush window of your program.
I really like when I use it but it gives only one color.here you can learn how to use this brush so you can have and other colors.
1.Open a new document and fill it with black color.we are going to use it as the background.
2.Go to the layers window create a new layer in transparency and add it to the image.name it green.
3.Go to tools choose green color then the bucket tool and fill the top layer.
[b]4.Set the green layer to opacity mode and select the background layer.
You can use any color you like for the overladed layer.you can use different colors from other brushes or you can fill it with a gradient to have the result you want.
seems i am the only one that didn't understand 4
what means "Set the green layer to opacity mode" ?