Optical mixing is the result of placing two different colors next to each other. The closer the colors are to complements of each other, the more the colors will stand out against each other. In the case of black lines on white paper, the black lines will be very visible, since human eyes will pick up on the border between white and black very easily. If you were to instead use thin yellow lines, it would be very hard to see, since the borders between yellow and white blend together so that we almost can’t see them at all.
how does optical mixing occur when thick black lines are used on white paper? – http://www.chacha.com/question/how-does-optical-mixing-occur-when-thick-black-lines-are-used-on-white-paper