Yren is one of those characters where something about his features just makes his expressions more subtle. Though sometimes even if it's harder to detect differences in a side-by-side, and static, comparison...they can be a little more obvious in game when you see the expression switch from one to the other.
Even so, the differences been serious, angry, annoyed, and sad...are still a little more subtle due to his eyebrow shape and other things.
Like Raif, I left his eyes in their 'neutral' state. But in reality, some of these expressions will likely be more closely tied to a specific eye colour.