Choosing the exact shade of a color is very difficult to do using a color wheel or "rainbow" color square.
ColorShades is designed to help you choose colors by looking at grids of related colors. You set the center color of a grid using the intuitive Hue-Saturation-Brightness model. You then pick which two out of Hue, Saturation, and Brightness that you want to vary across the grid, set the step sizes and ColorShades fills in the grid.
You can drag a color chip from any of the color rectangles in the grid and drop it anywhere on any application or color well that accepts color drags. You can also drag a color chip into the center rectangle of the grid to reset the center color.
Pausing the mouse for a few seconds over one of the color chips brings up a tool tip that shows the color coordinates (in HSB, RGB and a hex string representation of the RGB values) of the color under the mouse.