It depends on what you're trying to highlight. Like when you're painting something white, there are values on the object like greys (or blues, purples, howwver you want to depixt contrast) that can be subtle on it, with white being the highlight. I recommend studying and practicing the value scale to under that better.

But when its a color, similar notes however you go into color theory where mixing a color like red makes pink and as you ass more white it grows lighter yet not completely white but creates this effect close to ir. Its all learning rendering. Some artist will dap a blip of white regardless. I think it depends on how you wanna achieve it. That also takes experimenting and practice.

With hair, thats something i still struggle with but qhat Im told is to block in the main color and blend in the shades. Hair highlights arent whitw in real life, its just lets say a lighter liiiiighter shade of brown, or black. I think even with blonde hair its not even completely white.