Locate a picture of a "Sonic The Hedgehog" character online. Save the picture as a JPG or a PNG file.
Open the saved file with Microsoft Paint.
Select a color of choice from the color bar, normally located at the bottom of your Microsoft Paint application.
Select the "Paint Bucket" tool from the sidebar. The Paint Bucket tool is the icon that looks like a spilling paint bucket. You'll know that you have it when your cursor turns into the icon you just clicked.
Left-click inside the area you want to color. The heavy lines of most "Sonic The Hedgehog" art will contain the "spill" of new color within the boundaries. Repeat this process, changing colors as you see fit, until you're satisfied.