To those asking "Is this racism? Is that racism?" or saying "Why is it racism, when you could do the same to me and I wouldn't mind at all!":
I think you don't understand what racism is. It's a whole lot more than "Somebody singled me out in some way based on a trait I have that I didn't choose."
Racism is part and parcel of a system of oppression, and defining a light tone as "skin color" is part of the cultural aspects of maintaining that system.
And by the way, PC doesn't mean "I want to pass laws on what you can say and what you can't;" it means "I want to let you know what's offensive and why, because I think if you make an informed decision, you'll choose not to cause offense."
Anyway, others have explained it better, and in more detail:
"Racism refers to a variety of practices, beliefs, social relations, and phenomena that work to reproduce a racial hierarchy and social structure that yield superiority, power, and privilege for some, and discrimination and oppression for others."
"Racism involves one group having the power to carry out systematic discrimination through the institutional policies and practices of the society and by shaping the cultural beliefs and values that support those racist policies and practices."
"Racism = race prejudice + social and institutional power"
Maybe I should have added links to the sources of those quotes. But if you're interested, you'll look into it yourself. Just try searching for "racism is a system of oppression" or similar.