Nichols was only paid a fraction of Whitney's per episode salary (which was comparable to DeForest Kelley's), and only employed as a day player, not a regular. Whitney's release obviously didn't come down to any single issue, it I doubt it had much of anything to do with race.
If Whitney was being paid like a lead, when the show clearly had no intention of letting her play a lead, and she didn't have the race thing going for her, and she was the only recurring actress not "sleeping" with GR, then how the hell did she last 13 episodes?
Whitney's fate was probably sealed before they wrapped on her second episode.
She was intended to be a secondary lead like Kelley, which is why she had "Randcentric" episodes like Enemy With In
, Charlie X
. Whitney and Kelley get the same type of billing. When they let her go, her part went to one off characters like Helen Noel not Uhura. Whitney and Kelley get the same type of billing.