There are certain beauty lessons we all know: Dirty makeup brushes can cause breakouts, retinols and sunbathing don't mix, mascara needs to be replaced regularly, and so on. But the proper care and maintenance of blonde hair can be particularly elusive. Why? Because blonde hair is so damn difficult to take care of.One of the most common issues? Unwanted brassy and yellow tones. Luckily, you don't need a PhD in chemistry to learn how to keep the color from veering into that territory. For the CliffsNotes, we consulted two color experts: Riawna Capri , owner of L.A.’s Nine Zero One Salon and hairstylist to stars like JulianneHough and Nina Dobrev, and Joico celebrity colorist Denis De Souza , who’s worked with the likes of Olivia Wilde, Kate Mara, and Jenna Dewan.“Champagne blondes look more expensive and more natural," Capri tells us. "When your hair is yellow and brassy it looks cheap, like you haven’t been taking care of it — and it might not be your fault!”Why is blonde hair so prone to discoloration? There are two main reasons. First, understand that rarely is hair lightened to the exact shade that’s desired. Instead, once hair is bleached out, it must then be treated with a toner to create the sought-after shade. Capri explains it like this: Hair color is like leggings, and toner is like pantyhose. Since the toner your colorist applies is super-sheer, it can wear off quickly, leaving unwanted hues behind. That is, unless it's cared for properly.Second reason: “Because blonde hair tends to be more porous, it ends up discoloring over time,” De Souza says. Which means it's soaking up things that are messing with your blonde, like water and products. However, armed with the right knowledge and tools, you can keep your blonde from going bad.Click through for the top seven common discoloration culprits to avoid.