When Ghost hunt was published, it was in 1989. Assuming in 1989, Lin was 27, he would’ve been born in the 1960’s. During that time, Trachoma was at it’s highest peak. Lin might’ve caught the infection in his right eye, causing him to go blind.
Another assumption is that he’s colour blind, just like how some cats have one green eye and one blue.
Or, Lin was just born like that.
Or maybe Lin just has some harmless heterochromia.