Steward suffers from bad migraines, so one day Walter offered to come up with an eye serum that would temporarily reduce light intake. Steward, happily accepted but while he was helping Walter in the lab he made a mistake with the dosage.
Walter noticed - of course - but didn't feel the need to inform him.
Steward’s headaches became worse.
His right eye is now bright blue while his left one is still the deep dark bown the other one used to be aswell.