Sorting color is problematic. Yes, it is an easy first sort step with relatively limited number of output bins. But once sorted by color it becomes 10 times at least harder to pick out a shape. Anybody that I know that has sorted Lego in quantity first goes for that because it seems so obvious, and then after a while all of them revert.
Think about it: pick the 2x4 brick in red from a pile of 2x4 bricks in various colors vs pick the 2x4 brick in red from a pile of red bricks in various shapes. The first you can do in a heartbeat, the second will take much longer, and it is also much harder to determine that a piece isn't there in the first place.
Think about it: pick the 2x4 brick in red from a pile of 2x4 bricks in various colors vs pick the 2x4 brick in red from a pile of red bricks in various shapes. The first you can do in a heartbeat, the second will take much longer, and it is also much harder to determine that a piece isn't there in the first place.