Bro, I'll just tell you something. When you finish sketching something, there's like a paint bucket tool. Use that; it saves you time. For example, if you draw a cube and want to paint it green, just change the color to green and add the paint bucket. If it's done correctly, it will fill the cube. If it doesn't fill, it's because there's a small mistake. [Just so you know, you're welcome.]

Good

Ok