Probably because it is winter and the rest of your body is covered by long sleeves and long pants so it's not exposed to sunlight.
Assuming that you are a dark skinned race, the reason your face would be becoming darker than the rest of your body is because it is exposed to sunlight every day no matter how low or how bright the sunlight is, it changes our skin color regardless of race.
If you don't want your face to get darker, then you need to use an SPF of 15 or higher.
Assuming that you are a dark skinned race, the reason your face would be becoming darker than the rest of your body is because it is exposed to sunlight every day no matter how low or how bright the sunlight is, it changes our skin color regardless of race.
If you don't want your face to get darker, then you need to use an SPF of 15 or higher.