The Color Picker contains an HSV (hue, saturation, value) color wheel. The hue is the attribute of a color that allows it to be classified as blue, green, yellow, or red, and so on; the saturation is the amount of hue in the color; the value is the brightness, lightness, and darkness of the color.
The center of the color wheel is less saturated than the outer edge of the wheel. Each wedge in the wheel is a different hue. The slider bar on the left of the wheel controls the value.
Position the mouse on the slider bar, and slowly drag the slider vertically. Drag down to darken the value; drag up to lighten the value.
Click on the desired color. A small white circle on the color wheel marks your color choice so you can see where it is in relation to the other colors on the wheel. The area around the color wheel also displays your color choice.
If you select the incorrect color, click on Undo.