A very basic example of a color picker which uses 2 sprites, 1 script, 1 object and 1 room.
The hue and saturation of the color are based on the cursor's position over the colored square, rather than by using draw_getpixel. So all values of 0 - 255 are represented for both hue and saturation on the square and you don't get the same slow down as you would with draw_getpixel.
The same object draws and detects interactions with the value slider to the right of the colored square. This completes the HSV color picker.