I wouldn't worry about the colors, it's fine as is. It only becomes a problem when you are trying to challenge yourself into doing high quality work with low quality colors. Since you're just starting, you shouldn't be concerned with that. One thing though, on the plank, there is no perspective so it looks flat. Just add another diagonal line on the left hand side and then connect it to the rest of it, and make it darker. I wish I could show you what I mean, but I hope you undersatnd. Basically you just see a bit of the side of the plank to show it's 3d.