The original colour is displayed in a swatch at the top right. The colour that you have selected is shown in a swatch directly below it (for comparison).
To select a colour, first click on the central column that looks like a rainbow. The colour grid will then change to show many shades of that colour. Click on the shade you like within the grid and the colour swatch will change to this new colour.
The red, green and blue (RGB) values along with the hexadecimal value will also be shown for this new selection. You can manipulate these manually if you want a specific colour.
Once you are happy with your selection just click 'OK'. If you want to revert to the original colour then choose 'Cancel'.