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.