When you attach a detector (camera) to a telescope, the pixels usually don’t have color information. We have to force the color information on to the photo. This means almost every single pretty space picture you’ve seen was representative. That’s okay. Usually, astronomers want data about things in space and the color stuff might actually get in the way of that data. But everyone loves pretty space pictures and astronomers sometimes want to show off what they have found.
Creating pretty space pictures is fun. It isn’t so much science as art. Colors in astronomical images are not “what you would see” through the telescope – at least not necessarily. The first thing someone making pretty space pictures should do is embrace that this is a creative process. Try to make something pretty and don’t try to make it “accurate”. Since most astronomical images, are representative, you get to decide how to make the color add up in your images.
There are lots of ways to make 3-color images. There are very few easy ways.
There are some standard astronomy tools like SAO DS9, Aladin, etc. And there are freely available image manipulation programs like GIMP. I’ve put together a few tutorials to make things easier (I hope).