This was Bradley's third published book, originally published in slightly abridged format in 1963, later re-issued in 1983 in its complete form (which this version is).
Technically, this is a semi-juvenile, with a protagonist, Bart Steele, who has just graduated from the Space Academy, ready to return to his Vegan home, where his father manages a fleet of inter-planetary space ships. Interstellar travel is the sole province of the alien Lhari, and humans can only ride as passengers in cold sleep in their vessels, supposedly due to the inability of the human body to withstand the stresses of hyperspace travel. The Lhari have formed a simple relationship with the Mentorians, humans who have had a slight genetic shift that allows them to withstand very high light illumination levels. The Lhari, who are also color blind, normally prefer these high light levels, matching their home world's level of illumination. Bart, who is half Mentorian, can also see farther into the optical spectrum than normal, allowing him to see an eighth `color'. This provides the basis for the book's title, and plays a role in the final plot resolution. The Mentorians provide translation, color interpretation, and other services for the Lhari, setting them somewhat apart from the rest of humanity, who look upon them with some suspiscion.
The story revolves around Bart being co-opted to find the secret of the Lhari warp-drive fueling material by surgically changing his appearance so he could pass as a Lhari and having him ship out as a crew member on a Lhari ship that is home world bound. During the trip, he naturally finds that many of his Lhari crewmates are neither ogres nor saints, and comes to question the moral rightness of `stealing' this secret. Thematically, this book restricts itself to `different is not necessarily bad' and `the end does not justify the means'.
In general, a pretty standard space-opera plot typical of the late fifties and early sixties in science fiction.
Marion Zimmer Bradley is always good and this was was well up to her standard. An older book, but well worth reading.