And for anyone wondering where the name 3blue1brown comes from: Grant Sanderson has a condition called heterochromia, his right eye is 3 parts blue, and 1 part brown, like his logo.
I wonder how this is resolved for places that ask for eye color (various people identification registries). If the information could be accurate and understandable (from a short database column entry), it's a quite unique identifier.
Heterochromia of the eye is called heterochromia iridum or heterochromia iridis. It can be complete or sectoral. In complete heterochromia, one iris is a different color from the other. In sectoral heterochromia, part of one iris is a different color from its remainder. In central heterochromia, there is a ring around the pupil or possibly spikes of different colors radiating from the pupil.
So, in case of Grant Sanderson, it would be sectoral heterochromia.
You can see it for example in this video: https://www.youtube.com/watch?v=-bc9EWhmDZg