The neon-like glow of a color that escapes the boundaries of a real figure and spreads to where there is no color.
The exact causes of the neon color spreading illusion are not known. It seems to occur most often when black lines are substituted with colored lines on a white background.
The brightness conditions under which the color spreading figures are viewed change the perceived intensity of the effect. Under bright lighting the effect will be inhibited and under dim lighting the effect will be enhanced. (Wikipedia)
A blue inner circle appears within the black contours. In the actual image this area is white, which is made evident if the image is magnified.
Neon Color Spreading
Color effects: Another aspect of neon color spreading that can affect the magnitude of the illusion are the colors used within the illusion. Different colors tend to cause a less or more intense illusion. Changing the color of the background can also enhance or inhibit the effect. If contrasting colors are used, such as a yellow background with blue and black lines, the effect will be enhanced. If similar colors are used, the effect will be inhibited.
Blue and red versus green and yellow: Long and short wavelength light, where the human eye is less sensitive to spatial detail, seem to enhance the effect. This means that if the illusion is created with red or blue lines, black lines, and a white background, the effect will be more intense. This is particularly notable when the colors are more saturated. In contrast to this, green and yellow tend to suppress the effect of neon color spreading when used in the same way. (Wikipedia)
Ehrenstein figures are a good way of easily making persistent color spreading effects. They are good for showing both the neon color spreading illusion and illusory contours. They are also good for showing examples of differences in hue between inner and outer lines and how they affect the neon color spreading illusion. (Wikipedia)