If you look at a cyan colored ribbon under white light. What three primary colors is your eye not detecting?

Red

Explanation:

Cyan is a color that is formed from then combination of green and blue light. It is a greenish-blue color. It has a predominant wavelength of between 490-520 nm and wavelengths of between green and blue.

Primary colors consists of red, blue and green.

When Cyan is placed under white light, it doesn't detect the primary color red. It only detects blue and green.