A human being's anatomy and appearance depend on nature and nurture. Nature is what one is born as, and nurture is what his environment shapes him into.
To do justice to this question, one must travel back thousands of years. India was first inhabited by people of African origin who belonged to the Negroid race. They travelled from Africa, along the coasts the Middle Eastern countries, and eventually established what is known today as the Indus Valley Civilization in Northern and Western India. They gradually spread to other parts of the country as well. About 5000 years ago, the Aryans, a Caucasoid people, "invaded" India. This invasion pushed the inhabitants of that region to the south. So, a few thousand years ago, there probably was a distinct "white" India, separated by the central mountains from the "dark" India. But, over time, there would have been some intermingling due to which, dark skinned people would have settled in the northern parts of India and vice versa. They would have had kids with the fair skinned people, causing their features to be imparted to fair skinned kids and so on.
Suryansh is right in stating that climatic conditions have no role in deciding a person's colour. However, over a period of time, climatic conditions do bring about changes in genetic structure. Prolonged exposure to sunlight and heat can gradually increase the quantity of melanin. It shows up as tan temporarily, but over generations, the body tends to adapt itself to its surroundings, because of which subsequent generations show higher melanin content resulting in darker skin tones.
So, with one thing and another, we have dark people in North India and fair ones down south. Mongoloid people would have also tricked in from the Eastern parts resulting in the introduction of yellow skin tones as well. So in terms of chromo-demographics, we're fairly balanced as of now. However, as you travel from Kashmir to Kanyakumari, there is a fairly evident darkening of skin tones. The same holds true for people living in hilly areas as opposed to those who inhabit plains. It's not racist to say North Indian people are, on an average, fairer. In fact, it's only fair. Just that it's not as evident today as it probably was 5000 years ago.
Most of The Races are mixed up in India but in rural parts you can clearly see the difference.
If you see an Original who's family bell originally to kashmir or Himachal you will probably see the difference I am talking about here.
Rajasthan is central west india for Me.When I say North I just mean himalyan states like kashmir himachal
