Are all the following combinations of quantifiers correct?
a. People have different skin colors. Some are yellow, some are white, and some are black.
b. People have different skin colors. Some are yellow, some are white, and still some are black.
c. People have different skin colors. Some are yellow, some are white, and others are black.
d. People have different skin colors. Some are yellow, others are white, and still others are black.
All your examples are grammatical and understandable. But some are more idiomatic than others
Example b strikes me as quite odd. According to Ngram, “still some” and “still others” are acceptable, but the great deficiency in Ngram is that it ignores context. I doubt that in the context of a simple list “still some” is idiomatic. I cannot, however, cite an authority to support that assertion.
Examples a, c, and d are certainly idiomatic. In terms of style, example a is parallel but repetitive. Example c seems to sacrifice parallelism without eliminating repetition. Example d is better than c at eliminating repetition. Although you do not mention this option, “still yet others” provides even greater variety.
Recommendations about style are matters of opinion. Example a has the merit of not encouraging an inference that this is anything more than a list. With such a basic list, however, parallelism is probably not needed to clarify your intended thought, and example d has the merit of being less boring to read. It is up to you which choice you prefer.
