The Overlord's Thumb by Robert Silverberg

His choice would govern a boy's fate -- and, / incidentally, Earth's entire future.

The sun had gone down blood-red, and Colonel John Devall slept poorly because of it. The atmosphere on Markin was not normally conducive to blood-red sunsets, though they did happen occasionally on evenings when the blue of sunlight was scattered particularly well. The Marks connected red sunsets with approaching trouble. Colonel Devall, who headed the Terran cultural and military mission to Markin, was more cultural than military himself, and so was willing to accept the Markin belief that the sunset was a premonition of conflict.