The lines on the left artwork appear royal-blue and bright-pink, whilst on the right they appear blue-grey and orange-red. Whilst the blues are different, the bright-pink and the orange-red are exactly the same red. This can be explained in relation to simultaneous contrast.

Simultaneous contrast occurs for the physiological reason that our eyes are always seeking the complimentary colour to the colour we are looking at. Consequently our eye projects the opposite colour next to or around a coloured area, thus affecting the appearance of the surrounding colour. As the two blues are different in the two bowls our eye projects a different colour around each blue making the red appear different in each case.