I expect I haven't read enough SF/F to really comment well on this, but what precisely do people mean by writers or characters "of color"? In the States it often seems to be more related to culture than to skin color-- "acting white," for instance. In fiction, LeGuin's Ged, to name the most-cited example in the discussions I've read, is dark-skinned, but does not particularly seem like a member of any of Earth's dark-skinned races. He's not particularly tribal. He's well-educated, powerful, ambitious (at first) and individualistic, all of which characteristics he shares with Madison Avenue executives. (Others, of course, he doesn't.)
Point being, what is the real significance to being dark-skinned? If most prejudice is actually cultural in nature, is there much value in revolting against prejudice based solely on skin color? Missing the target, as it were.