Should this be xanthochroid (with no "r" after the "th")? And would it mean, then either "yellow-skinned" (jaundiced? East Asian?) or "pale yellow"? Presumably Pynchon is using it in the sense of "sun-bleached blond".
This spelling is an error for 'xanthochroid', which can be analysed as coming from either ochr- "pale" or chro- "skin". Huxley, the coiner of the original noun 'Xanthochroi', apparently intended the former (according to the OED).