I think the point is that he cut himself off from others to the point where he never made the effort to understand their feelings. Could have been portrayed more clearly, though.
That part was understandable. But I was just wondering about the typical things that don't require deep understanding. Such as walking to the market and seeing a person crying their eyes out and their aura is deep blue. Deep blue could then have the feeling of sadness. Just the general emotions you see. Maybe he could have witnessed a fight between two lovers and their aura show dark green. Who knows, any obvious situation.
I feel like Kahara should have a better grasp of the colors and their meaning considering he has lived his entire life with the ability so he has probably seen a ton of situations where different colors show up. The fact that he didn't know dark green meant jealousy makes me wonder if he Kahara walks everywhere with his eyes closed only to open them at school, at the daycare, and at home. There are a ton of times he could have seen various colors multiple times and that can help him put meaning to them.