R. Star wrote:
... and the plot summary is "white woman kidnapped on planet full of black people!"
If that was the plot summary, it would have been misleading. Yar was kidnapped not because she was white, but rather because she had just put on a brief (and not very impressive) martial arts demonstration, and Lutan thought manipulating Yar into killing his wife would secede, where his previous attempts to kill her using others had failed.
Yar's "white'ness" had nothing to do with her selection.