Purple is a color associated with royalty, but red is a noble color. Maybe not royalty, but lords families and such. Two of the most common colors in crests are Red and Navy. Red is also a color seen on most countries' flags.
And red eyes are associated with evil because it's unnatural. Like, there are plenty of people with eyes that are blue and brown, and there are some with green, and a few people have eyes that look almost purple. Red eyes are impossible. Except if you count albinos. But albinos were thought to be possessed once. So that could also lend to the evil eyes are red thing.
