Answer : Green
Green is complementary for colour of red .
When organic molecules are exposed to radiations in the visible region, they may absorb a certain portion of visible light. If a certain portion of the light is absorbed and the rest is reflected, the substance has the colour of the reflected light. All dyes absorb a certain portion of visible light since they are organic compounds. The colour of light reflected which is complementary to the colour absorbed, imparts colour to the dye.