If it's discolored due to muck, I personally choose to toss as it means it sat in wet muck and thus more likely bacteria broke through the bloom barrier.
Agreed...or it could be coloration picked up from mud or muck in the coop since it was pushed aside. I've had a number of them laid in the yard take on coloration from the clay soil or even in the nest boxes as the hen tracked mud in on her feet, then laid the egg.
Here's a nifty egg chart for...