When Purple Perceived Only at Fixation: A Fixation and Distance-Dependent Color Illusion

Are these dots purple or blue?

Here are two optical illusion, wherein purple structures are perceived as purple exclusively at the point of fixation, while the surrounding dots transition towards a blue hue. As the viewing distance increases, a greater number of dots revert to a purple appearance, culminating in the pattern being perceived as uniformly purple.

9 Purple Dots Illusion — **9 Purple Dots** Move your gaze from one to another dot. The one you are focusing on appears purple while the other perceived more blue. (c) H. Schulz-Hildebrandt

Color Changing Dots Illusion — **Color Changing Dots** When changing the distance to the monitor, more or less dots perceived as purple. (c) H. Schulz-Hildebrandt

This effect is best visible on a cellphone display. It will be less obviously when the color temperature of the display has changed (NightMode, Dark mode etc.)

Ho does this (likely) works?

First, a classic color contrast effect, which would explain why the periphery structures show up in blueish, second, the reception of purple as a non-spectral color that is created exclusively through the combinet stimulation of red and blue receptors, and third, the center of our vision is less sensitive to blue light. The combination of these three effects aligns with the perception of the optical illusion which only occurs reliably in a narrowly limited color space and is clearly dependent on the viewing distance.

Try it yourself

Perceptual Color Test: Decide whether each color set shows a visible shift or not. (Link)
Color-Matching Task: Adjust the blue component to find when center and outer colors appear identical. (Link)

Both links let you explore how your own color perception varies.

Hinnerk Schulz-Hildebrandt