It varies wildly. For example, in the Movie X-Men, Rogue's hair is turned white during the ordeal with Magneto's machine. In certain comics, she dyes/bleaches it white because she likes it. In others, it's because she absorbed a portion of Magneto's life-force, and that turned her hair white. Coloring the sides of a man's hair, similar to your Nick Fury & Mr. Fantastic, is also a common way to show a man has "aged well" even though his face & stature hasn't really changed. This is largely due to men, both real and comic-bound, having a tendancy to have shorter hair on the sides of their head in relation to the top.
In the real world, it is common for a person who undergoes trauma or intense stress over an extended period of time to have a portion or all of their hair to turn white (referenced in the X-men movie above). It isn't usually something that happens overnight, or even over a small period of time, but it can happen. People have even pointed to pictures of President Obama before election and current to show how much the stress of the presidency has taken it's toll. Being a superhero, or villain, would probably also take a similar impact on one's body.
In general, media guidelines suggest that when you have bodies of the same shape or size, as a general rule, you want to create visual cues so the observer (reader, watcher, player, et cetera) so you don't have to constantly give a full face shot so the observer can identify him or her. Sometimes this manifests as a specific outfit (for example, the Power Rangers also wearing their specific colors when not actually in their ranger suits), as an overarching theme of a character (Ex: Both Nidalee and Kalista are "relatively scantily clad, spear-throwing adult female" characters in League of Legends, so they not only have different color palettes, but also different ways of moving, and different roles in the game). Little things like having a unique haircuts/styles or signature clothing help observers keep tabs on who's who without extra effort. And the more that unique object "pops" against the background, the better.
For example, "Busty flying white girl in green leotard with yellow accents" could be Meggan, Siryn, or Rogue (or, or, or...), so the artists use things like hair color to help tell people apart.