are you using Paint ? from windows ? because that program is pretty hard to color digitaly , you can try the paint bucket. But if you are looking for a not-so-expensive program , you can buy PaintToolSai, you can use it for a month in tryouts ! and its very easy to color with ! all you have to do is click on a '(what they call magic wand) and tap it in the place you want colored in, then you go to the layer where you want to clolor it ! and select your color and you will see it fill in all by itself !!!
