Ethnicities of Pakistan

Phenotype refers to an individual's observable traits, such as height, eye colour and skin tone. A person's phenotype is determined by both their genomic makeup (genotype) and environmental factors.

It is important to understand phenotypes because it gives you a better understanding of the historical migrations of people around the globe. It also helps understanding the topics of  admixtures, haplogroups, environments, and genealogy. 

Genes have a vital role in phenotype development, but they are not the sole factor. Phenotypes are influenced by environmental factors like temperature, light, heat, humidity, and nutrition availability, and so on.

In general, people living close to the equator are highly darkly pigmented, and those living near the poles are generally very lightly pigmented. The rest of humanity shows a high degree of skin colour variation between these two extremes, generally correlating with UV exposure. The main exception to this rule is in the New World, where people have only lived for about 10,000 to 15,000 years and show a less pronounced degree of skin pigmentation.

The closer you are to the equator, the more dark skinned you will be because the sun is closer and more direct, and more intense than any other area. So the body produces more melanin (skin pigment for dark shade) because melanin protects the skin from harmful effects of sun rays. The people living away from the equator receives less sun rays, so there's no requirement for the body to produce melanin.

The Indian Subcontinent has a micro climate right across it, varying from extreme heat in the south (nearer to the Equator) to the extreme cold in the north.  This takes us back to the environmental factors again. Also, the other factors such alleles plays a role of long term gene variation development through external factors such environments etc...