Function of colored tabs on SPD-SL Cleats
The coloured areas work as wear indicators, and the colour (e.g. yellow) tells you how much float the cleats have.

Looking at a pair just now, it seems that coloured areas assist the cleat in disengaging from the pedal. The front of the cleat acts as a pivot around which you twist the foot to release. The more material that remains here, the better it can act as a pivot.

The two rear areas of coloured cleat seem to ramp upwards against sloped areas on the back edges of the pedal when you twist the foot. This raises the back of the cleat which helps to open the spring mechanism of the pedal to release the cleat. If these areas wear down, they will not rise as high and so be less effective at releasing the cleat.

Edit: the cleat starts opening the spring before the rear coloured tab ramps up, but this ramping up gives the final 'pop' which releases the pedal.

Edit again: it is the inboard side of the rear coloured tab which ramps up, which would be why you have had no trouble yet clipping out when they are worn on the outboard side.