I was just wondering, because I see used pedals on ebay that frequently have the LED switched out for a different color. I know it probably sounds stupid, but I was just curious. I would hate to buy something and find out it was different because of a stupid light.
If it does, you wouldn't notice it. LEDs require a very low level of power (not sure how much exactly)
Maybe it's just for visualization? I mean, sometimes it's quite dark on stage and another colour might be easier to see

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No difference on the sound, it just makes them look cooler, more unique.
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Actually, LEDs can make a difference in sound when placed in the circuit of an amplifier. However, in the case of a pedal, they're only for aesthetic purposes.
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as Raijouta said, it really depends on how the LED is placed in the circuit. some pedals use LEDs as clipping diodes, so changing them out will change the sound. in some cases, it changes the sound quite drasticly. things like keeley's SEM (think thats the one) have an external LED that is part of the circuit. when you play, it lights up. so aesthetics and it changes the sound.

however, usually the main LED in a pedal isnt in the circuit. so it wont change the tone.