Since you've thrown everything at it except the kitchen sink, my guess would be that it's genetic. In Japan and China, Clivia breeders have rigorously selected strains which exhibit various "chlorophyllic abnormalities" and charge premium prices for them. Search on keywords Clivia, Akebono, Light of Buddha, Painted Face for examples.
As far as vigor goes, has it kept pace with its green siblings?
