Color changes when I visit my hometown.

I have gernal questions which I can't find answers on Google. I am from a hilly small town of Uttrakhand but currently I live in Delhi. Whenever I visit my hometown with in a week I become super dark and when I come back I return to my normal skin colour and it's not just me but anyone from mountains community have the same effect less or more depending on person. Same thing happens with the person living in hills when they visit Delhi they become super light and after a month or so they return to normal color. It's not just Delhi but any city near near my hometown those are in valley has the same effect. Could it be bcz of the sun. People of community says it's the water effect however this effect doesn't last more than a month and people return back to Thier normal color.