The Colors of Space by Marion Zimmer Bradley (little readers TXT) 📕

Bart felt the involuntary surge of anger, instantly controlled. "It's not that way at all. My mother was a Mentorian, remember. She made five cruises on a Lhari ship before she married my father."

Tommy sighed. "I guess I'm just jealous--to think the Mentorians can sign on the Lhari ship as crew, while you and I will never pilot a ship between the stars. What did she do?"

"She was a mathematician. Before the Lhari met up with men, they used a system of mathematics as clumsy as the old Roman numerals. You have to admire them, when you realize that they learned stellar navigation with their old system, though most ships use human math now. And of course, you know their eyes aren't like ours. Among other things, they're color-blind. They see everything in shades of black or white or gray.

"So they found out that humans aboard their ships were useful. You remember how humans, in the early days in space,