Paint Tool Sai much better than paint?

Normal Paint on Windows 7 just isn't going to cut it. I want to color my anime drawings professionally, with shading and blended colors. I'm not worried about doing it because I can do it on paint, but it takes me weeks to do little portions because the stupid fill only works on things that are the same color. I want something that can fill in a full selected region, so I don't have to sit on the computer for hours, manually coloring with a paint brush. I have to zoom for accurateness, so you can imagine how long that takes. I don't think I've ever finished a picture because of it. I either scan or take pictures of my drawings and upload them to the computer. The problem is that the paper shows up not all the same color, most likely from graphite smears, and I wouldn't expect the paper to be fully white in the first place. There are tons of variations of grays that paint picks up, so that completely ruins the use of the fill tool. It's driving me insane. I know I can just use colored pencils, which I do, and they come out really nice, but I'd like to use the computer to get a smooth and really finished look. Can I do that with Paint Tool Sai? I don't want to waste my money on something that can't even do that. Thanks.