Explore how you perceive colors and intensities in relation to their surrounding color(s).
You can set the background colors and intensities by clicking on the color boxes and sliding the scrollbars on the left and right sides of the display.
You can set the foreground color and intensity by clicking on the color boxes and sliding the scrollbar at the bottom.
Finally, to make it more interesting you can change the background from solid colors to a checkerboard pattern by checking the checkbox, and notice how that also changes your perception! When the box is checked, the background colors (shades) are set by the same sliders. The other slider value is used to put a checkerboard pattern on top of the box, so you can still make the box "disappear".