Color (слэш)

Set in an alternate universe where everyone sees in black and white until they meet their soul mate, which gives the human eye the ability to process colors. One of the world's greatest tragedies is when someone is color blind because even if they meet their soul mate, they will still only be able to see in black and white.
Cue Sherlock and John, who always seem to attract whatever problem exists in whatever universe they're written in.