I disagree with some of the other commentators about buying Photoshop. I do my comics exclusively in Photoshop now because that's what I'm used to. But Photoshop is expensive as all hell and it has a lot of features that I don't really use. If you're considering buying a better art program, I'd advise you to shop around a bit more. Look at other programs and always download the free trial before buying the whole program.
I don't think you necessarily need to buy a new expensive art program right away, but I would like to see you experiment with color. Don't be afraid to try a bunch of different techniques (digital or traditional) and see what sticks.