There is no blond option for a hairstyle for a specific sim.

Product: The Sims 4
Platform:PC
Which language are you playing the game in? English
How often does the bug occur? Often (50% - 99%)
What is your current game version number? whatever the latest one is
What expansions, game packs, and stuff packs do you have installed? All expansions but Get Famous and Dine Out
Steps: How can we find the bug ourselves? It's strange because it only happens with my one sim, if I try it on a different one the bug does not happen
What happens when the bug occurs? My sim is blond and I want to change his hair to one of the Discover University ones and when I switch it, his hair color changes to black and there is no option for his original hair color for that specific hairstyle.
What do you expect to see? To change his hair and there is a blond option
Have you installed any customization with the game, e.g. Custom Content or Mods? Not now. I've removed them.
Did this issue appear after a specific patch or change you made to your system? Neutral/Not Sure 

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So I go to change my blond sim's hair and I click on one of the Discover University hairstyles and his hair changes to black and there is no blond option for that specific hairstyle. I tried it with a different sim and there was a blond option for the hairstyle.