What are the function of the colored tabs on the SPD-SL cleats?
If they wear through to the black, Shimano recommends their replacement.
In my own case, the outboard side of the colored pads are worn down, but not the front or the inboard side. To me, the function of the front is obvious, it is what actually clips you in. However the sides are a bit of a mystery.
Shimano also says that if you have trouble clipping in or out, or staying clipped in then you should replace the cleats. I have had no such troubles despite the pads being worn down.