How we perceive smaller the areas of the (colourful) rings than of their inner (white) circles

0.00.10.20.30.40.50.60.70.80.91.0Real values of percentage of colour-0.5-0.4-0.3-0.2-0.10.00.10.20.30.40.50.6Errors of Estimates

Estimates of percentage of colour in the 4 circles above (i.e., number of black pixels vs. number of black and white pixels). Experiment on 45 university students.

The values on x-axis are slightly randomly moved, so the overlapted points may be visible.

(One group was asked about the percentage of black, the other about white - no significant difference)