Are all the following combinations of quantifiers correct?

a. There are different skin colors. Some are yellow, some are white, and some are black.

b. There are different skin colors. Some are yellow, some are white, and still some are black.

c. There are different skin colors. Some are yellow, some are white, and others are black.

d. There are different skin colors. Some are yellow, others are white, and still others are black.

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.