Contrasts produce contours
We call color contrasts the differences that increase the visual perception;
Here are six:
Hue contrast (color per se)
Hue contrast: at its highest between complementary colors, just like green on the big red square.
Luminosity contrast: the same reference color mixed with more or less white and black.
Saturation contrast: a saturated blue as a reference (B square, R=0, G=0, B=255), randomly mixed with greys and other colors.
Simultaneous contrast: this contrast is in the grey looking different according to the background color (it gets a "ghost" color). Simultaneous contrast is always at work as it occurs everytime 2 colors are adjoining: as the color changes background, the ghost color changes.
Warm-Cold contrast: so called warm (from red to yellow) and cold (from green to blue) alternating colors.
Size contrast: rather obvious, it is mostly detected as a pattern on a background.
Playing with saturated colors contrasts - greys in the right column.
Do not confuse colors contrasts with visual contrast; the latter is at its highest between yellow and black (i.e. street signs); it sums hue, luminosity and saturation contrasts.