Click the 'Pick From Screen' button and it will display the color under the mouse cursor. Click again to 'pick up' that color.
The output is shown as 3 trackbars representing the red, green and blue components of the color, and there are two labels showing the color as HEX (HTML) and Decimal.

There are also options for point sample, which is the color of the pixel directly under the cursor, 3x3 average sample, which is an average of the 9 pixels around the cursor, and similarly a 5x5 average sample. When using the 3x3 or 5x5 average, a magnified version of the pixels being sampled is displayed in a panel on the form.

Download full source code.