Isaac is supposed to be 700 years old, but that might be young by Kaylon standards.
The red and orange eyes let us know that the other Kaylons aren't Isaac. Him having the only blue eyes makes it easier to pick him out of a crowd in some of the scenes. So the eyes are there for the audience. But since Isaac has shared everything with his people on the homeworld, they would know that eyes are important to humans, so they might have just put them on for the sake of their visitors. Isaac probably chose blue because he wanted to comfort the crew members and appear non-threatening. But maybe on the homeworld they wanted to intimidate the Orville crew, so they chose bright colours.