On Q2, as far as I know the answer is as per the previous posters - it can be done but it's not worth the considerable effort involved.

On Q1, the answer is subtly different - it can be done and it's not hard but it's still not worth the effort. The colour is due to the presence of a thin oxide layer which refracts light. The oxide structure has a different atomic spacing from the parent metal and this causes the surface to be placed in tension. The residual tensile stress means that a small crack or scratch is more likely to continue to expand, so the effect is to weaken the material.

Paint, on the other hand, doesn't have this problem.