Simultaneous Contrast

I was teaching a student the other day and we were having a problem.  She was mixing color to use in painting an antique wood box that was a support for a still life setup.  There is a wood fired clay canister that has a beautiful yellow green glaze and several antique copper pieces in the still life.  I had her do a burnt sienna underpainting to complement the green glaze and help with the copper.  I showed her how to mix a color using yellow ochre and burnt sienna for the wood box and she got a nice warm yellow brown.  She put it on the canvas, and it was green.  Hmm...I had a suspicion we were dealing with simultaneous contrast.  But, the effect was more striking than any I had seen before, I wanted to be sure.  I had her clean the brush, same thing.  I tried another tube of yellow ochre, same thing.  I tried a different brand of yellow ochre, yet again it appeared green against the burnt sienna underpainting.  A very cool opportunity to illustrate to my student the concept of simultaneous contrast.

The original color, a mix of yellow ochre and burnt sienna, on the palette

Notice how the yellow ochre  looks green when applied over the burnt sienna underpainting

Simultaneous contrast is how two colors (or two values) next to each other can change how our brain perceives them.  Many optical illusions are based on this effect.

Click here to see some simultaneous contrast illusions based on value.
Click here to see some simultaneous contrast illusions based on color.

To overcome this problem, I had her use a mix of burnt sienna and cadmium yellow medium, which gave her the color she was looking for.  We will use the effect of the yellow ochre appearing green against the burnt sienna to good effect when it comes to painting the pottery canister.