For example, Meyer made the following experiment.

He covered a piece of green paper by a sheet of thin transparent white paper.

The colour of this double surface was, of course, a pale green.

He then introduced a scrap of grey paper between the two sheets, and found that, instead of looking whitish as it really was, it looked rose-red.
Whatever the colour of the under sheet the grey scrap took the complementary hue.

If, however, the piece of grey paper is put outside the thin sheet, it looks grey; and what is most remarkable is that when a second piece is put outside, the scrap inside no longer wears the complementary hue.
There is here evidently something more than a change of organic conditions; there is an action of experience and suggestion.