Abstract: Apparatus for mixing granules of different colors for use in surfacing for example asphalt shingles in which the valve opening is controlled automatically, the valve being opened by a drive having a fixed valve opening travel and the different valve open positions being determined by intially presetting the valve opening drive by operating it a predetermined amount from a home position in the direction opposite from the valve opening direction, reversing the drive motor to return it to the home position while recording the motion as a part of the fixed valve opening motion, and reenergizing the drive to complete the valve opening motion. The apparatus is also adapted to be operated manually in the event of a malfunction or power failure and it is provided with diamond-shaped orifices whereby the valve opening is a function of the square of the percent of valve travel to provide more accurate control in the low valve opening ranges.