Color Picker

Color Picker
Access color picker by clicking on color indicator located on the left toolbar, or by pressing F key to toggle.
Color square represents current color in picker. It is located on the outside of the color picker menu. You can position square in regard to canvas, to visually approximate how the color will fit.

Eyedropper Tool is used to sample color from the canvas.
Hold Shift key and click on the canvas to sample color. Alternatively, if the Shift key is reserved as a modifier for a particular tool - use Eyedropper Tool hotkey: D
Dropper layer mode is an option available in Color Picker menu, and allows for Eyedropper Tool to sample color from selected layer at maximum opacity.
e.g. If a layer has black color semi-transparent and it looks gray on white background of a layer underneath - sampled color will be pure black. The idea is that this option should help you with transparent overlays by giving you absolute color of the region.