If you've ever seen Path of Exile you probably noticed that all its gear has red, green, and blue circular sockets on them. Those sockets are the basis of its character build customization: You don't learn skills in PoE, you pick up skill gems and insert them into your gear. Skill gems–and multi-colored sockets—have been core to PoE ever since it was released.
They're also the first roadblock for new players who aren't coming into every new league, or season, with a plan. I remember playing PoE years ago and getting annoyed that I couldn't fit a skill gem into any of my items because the colors didn't match. There are ways to get around this, of course, but it's a struggle for everyone at the start of the campaign and a constant bit of friction as you upgrade your gear.
