Zeno is backstage at the school play, setting a prop table under a pure blue stage light. The director's red apple prop is on the table.
Under the blue light, the apple does not look dim red. It looks black. The color of an object is not glued to the object, and this lesson is the two-step read that predicts what you will see.
A surface sorts whatever light arrives: it reflects its own set of wavelengths and absorbs the rest. Your eye receives only what the surface reflects.
Under white light, every wavelength arrives, so the apple's skin always finds red to reflect. That is the only situation you have seen so far.
Under a single-color light, only one set of wavelengths arrives. Now the sorting has less to work with.
If yes, the surface reflects them, and you see that color. If no, the surface absorbs everything that arrives and reflects nothing.
A surface that reflects none of the arriving light looks black.
So the red apple under pure blue light looks black: the skin reflects red, but no red arrives, and the arriving blue is absorbed.
The apple did not change. The light did. What you see is always the pair working together: the surface's reflected set, and the wavelengths the light supplies.
A green leaf sits under pure red light. Predict its apparent color.
Step one: the leaf's surface reflects green wavelengths.
Step two: pure red light carries no green.
The leaf absorbs the arriving red and reflects nothing.
A surface that reflects none of the arriving light looks black.
The leaf looks black.
A white sneaker sits under pure blue light. Predict its apparent color.
Step one: a white surface reflects nearly all arriving wavelengths, whatever they are.
Step two: the arriving light is blue.
The sneaker reflects the arriving blue.
The sneaker looks blue.
A blue cap sits under pure blue light. What color does it appear?
Bard says: 'A white shirt under pure blue light looks blue.' Is Bard correct?
Ysolde says: 'A red apple looks red under any color of light, because the redness is in the apple itself.' Is Ysolde correct?
A yellow raincoat hangs under pure blue light. What color does it appear?
Amity's green bike stands under pure red light and looks black. Why?
A T-shirt looks green under white light and black under pure red light. Which wavelengths does the shirt's surface reflect?