@WhimsiCat with his comment, that seemed pretty obvious to me already.
What I don't understand is why the amber eyes are so looked down on. Yes, it takes a few years to learn to use magic. But they can then use ALL elements, instead of just one or two like the 'common' mages.
I would totally be willing to study magic for a decade to get the incredibly versatile multi-magic over just being able to...say...burn things better.
Edit: After re-reading the beginning chapters, I realize that people with amber eyes aren't even being taught magic anymore... Which seems incredibly stupid, considering he said that 90% of the population has them. That is a LOT of mages that will never be valuable...