The reading for 髪 isn't the one you learned. It's actually はつ, but it's getting rendaku'd here. So, to remember the reading of it for this word, just think about how if you were blonde, you'd wear many hats (はつ) to hide it. I have no idea why you'd hide it, but you can come up with a compelling reason that makes sense to you.
I wanted to be blonde so I bleached my hair with Coca-Cola.