The Colors of Space

Humans have spread to other planets within our galaxy, partly helped by a race of apparently friendly aliens known as the Lhari.

They run a faster-than-light (FTL) transport network in human-occupied space, using a technology based on a rare substance from their own galaxy.

The protagonist, Bart Steele, is recruited by an illegal organisation to act as a spy on board a Lhari ship.

The circumstances of the confession put Steele in danger, which helps to convince the captain that he genuinely does not wish to harm them.

Steele remembers seeing that same color while visiting an uninhabited planet on his initial voyage, and realises that he now knows the location of a source of fuel within our own galaxy.