How come when I color my drawings, it ruins it?

Think of it this way: You had to practice a lot to get good at drawing, right? Well, the same thing happens with coloring. It takes a lot of practice to not make it look juvenile. So just practice a lot, and besides, colored pencils take a lot of work to not make it childish, so you can try oil pastels, but just be patient and stick with it. You'll get a lot better!