Hair is pretty much a easy thing, right? Just choice a random color of the rainbow, add a hairstyle, and BAM! Your good to go! Right? Wrong! Hair color and style can help your OC appear more realistic, especially if you match the hair to the correct timeline.
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For example, Hair Styles can serious out-date your OC if it's not in the correct timeline/era. You don't want to give your 1980's Stranger Things OC a hairstyle that would better match a Federal Japan geisha girl. If your OC is in a fandom, then check out the hairstyles of other Canon characters; see what hairstyles they have and go from there. I'm not saying to copy a Canon characters hairstyle (especially if there hairstyle is unique to that Canon character), but this is a good way to make a hairstyle for your OC. The same goes for OCs in original book; research the timeline that your universe is based off of and go from there. Reasearch is super important!!!!
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Hair Color is also a major thing! Not too many people have rainbow dyed hair (unless your Rainbow Dash from My Little Pony). Most people do have natural haircolors (such as browns, blonds, redheads, etc), though some people do decide to dye their hair (such as Jacksepticeye's green hair and DanTDM's blue hair).

When making a OC, I will use Google to look up hair colors; there are different shades of every natural or unnatural hair colors. You don't want to confuse someone when you intended your OCs blonde hair to be the shade of Rapunzel from Tangled, and not Elsa's hair color from Frozen. If you look at both characters, you'll notice Elsa has a lighter (almost white) shade of blonde hair (called platinum blonde) compared to Rapunzel's golden blonde hair.

Some people have 2 colors in their hair; for example, if you look at certain pictures of Markiplier, he will have both dyed red hair and his natural brown/black hair. Rogue from the X-Men has a streak of white hair in her natural brown hair, making her stand out from most of the other female X-Men. If you can't decide whether your OC should have dyed hair or not, then take these 2 example ideas and give your character both haircolors!

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Just please make the hair color and style make sense, ok? I can't tell you how many times I've seen people with OCs whose haircolors are out of this world (literally)! Make sure the haircolors and style makes sense for your story!

Examples :
X-Men mutants can have dyed hair and crazy hairstyles due to their mutation or dyeing/styling their hair
Stranger Things OC have (mostly) natural hair color and hairstyles based off of 1980's style/colors. If there hair is dyed, then most dyed hair is near-normal (that I know of).

If you don't know what hairstyle/color a OC could totally rock, then please do your research. (I'm gonna stop now before I run up a wall, lol).