How is it possible that the variety that we see amongst us today, in terms of color etc., can come from one person - Adam?

The geneticists, those who specialize in genetics, explain this very easily. Every human being has, in his genes, the ability to produce every other human being. Every human being, not just Adam-the first, but even we today have the ability to produce children of varying colors, shapes and sizes, etc. It is just a matter of factors leading to turning on some genes and turning off some other genes, and only God controls that.

One of the signs He leaves amongst us are the Albinos. One of my friends is a Sudanese. His parents and his grandparents are black but he is white as snow. So much so that his eyelashes are white. This is genetic. Prophet Muhammad (ﷺ) alluded to this fact in one of his hadith, where he said that when Adam (عَلَيْهِ السَّلاَمُ) was being made (from clay), the angels gathered from all over the earth, different amounts of dirt - some of it white, some black and some brown, as we have all this variation in colors of the earth itself. That was all brought together in the creation of Adam.

Further reading:

For details on the creation of Adam, see Tafseer Ibn Katheer and Tafseer ul Qurtabi [Surah al-Hijr, 15:25]