Model is multiple colors and renders with artifacts A couple days ago I decided to try Blender. 


Why on screen do some faces have a different color and some "scratches"? How do I make all of the 3d model one color?
And after rendering I have some artifacts (see screenshot). What is the reason of this?
There are many different reasons, a few fixes:


In edit mode, Press A until everything has been selected. Press Ctrl+N, This re-calculates normals and corrects them.
Press T Inside the 3D view, and on the left you should see a button that says "Remove Doubles", click that and it could possibly fix your problem.