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It won't look red if you put that sheet of glass over a black piece of paper.

Ink is semi-transparent. So think of white paper as a light source, and the red ink absorbs all colors but red coming off the paper before it reaches your eye.

Nail polish isn't transparent, but it still looks red because it simply reflects red light. That light never reaches the paper -- it bounces off the nail polish and comes back to your eye.

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