Shaving your upper lip will not make it darker although it can appear darker or thicker.
In reality, the shaving of your upper lip does not cause the upper lip hair to become thicker or darker although it can appear that way.
The appearance of the upper lip hair looking darker or thicker is often a result of the blunt tip of the newly shaved upper lip hair.
It can feel coarser and more noticeable as the upper lip hair grows out.
But shaving the upper lip hair or even lower lip hair or any hair for that matter does not change the growth rate of the hair, the color of the hair or the thickness of the hair.
It's a common misconception that the hair grows back darker or thicker.
When you shave, though you cut the hair at an angle, and that leaves a blunt tip to the hair.
This can make the new hair growth feel coarser and also appear darker or thicker as it grows.
But shaving the hair including your upper lip hair or lower lip hair or any hair does not change the growth cycle of the hair or the amount of the hair follicles.
All the hairs that are in an area will begin growing from the same point, but it doesn't mean the hairs will be darker in color or thicker.
However, while shaving does not cause the hair to darken, aggressive shaving of the hair can cause skin irritation, inflammation and even possible hyperpigmentation, which is darkening of your skin over time.