Observation: red and green lights make yellow; green and blue lights make greenish-blue…

Hypothesis: additive mixtures of lights average out the wavelengths to an intermediate colour.

Prediction: red and blue together will produce green.

Experimental: nope – it made purple.

Now, audience participation time – choose your own Conclusion!

  • colours add by a mechanism somewhat more involved than their position along the visible spectrum.
  • purple must be fake.
Chromaticity diagram of the entire range of human vision. Purple is included.
(spoiler: colours add along straight lines in this diagram)