This is to check that foreground and background color combinations provide sufficient contrast Two colors provide good color visibility if the brightness difference and the color difference between the two colors are greater than a set range. The range for color brightness difference is 125 and the range for color difference is 500 (refer http://www.w3.org/TR/AERT#color-contrast). Some may choose 400 to be the range of color difference as it may already provide sufficient contrast. These values can be changed at the Preferences Dialog - Color Analyzer.
Color Contrast Settings
Show the values of the color brightness and color difference that are in use.
Show the sample text using the foreground and background colors. If the color brightness and the color difference reach the preset values, a tick image will be shown otherwise a red cross will be shown.