there is a fairly simple way to do what you want using the GIMP. you want to fill in areas that are all one colour with a specific patter, yes?
what you'd need to do is make images with small sections of the patterns you want to use so that, if it were tiled repeatedly, all the lines would match up at the tiles' edges, and then save it as a GIMP pattern (*.pat) file and copy it into the pattern folder wherever you have GIMP installed. from then on, it will appear in your list of patterns. then use the select by colour tool to limit your selection to only the colour you want to replace with the pattern, choose the bucket fill tool, set it to "fill whole selection" and "pattern fill" and you can fill the entire area with that pattern.
