The second color deception exercise in the Albers/Nelson curriculum is to make 3 colors appear as 2. In this case, there are two ground colors, and the figure color is chosen such that on each ground, it appears to be the same color as the opposite ground. That’s pretty abstract – here’s what it looks like:
Again, the grays were taken from my array in this post.
For this deception to work, the ground colors need to be closely related, and the figure color must be right in between them.