About two ish weeks ago i put toothpaste under my lip for pimples but it turned out there was a mosquito bite as well and now my skin is brown and dar?

Unrelated: Toothpaste under the lip (or anywhere else) does nothing for pimples. (Wherever did you hear such a thing??) It also will not cause cany change in skin color. That has nothing to do with the toothpaste or the mosquito bite.

Toothpaste: Toothpaste is formulated to be used in the mouth, not on the skin. It can, in actuality, cause skin irritation with a reaction similar to a burn. Please see your Physician or a specialist Dermatologist for appropriate care. Please do not get your skin care instructions from a Google Search, your friends, or a non-professional YouTube video.