Methemoglobinemia also causes the pt to be cyanotic, quite dramatically in some cases. If you somehow manage to miss that it is probably time to find a new job.
I see you must live in an area where everyone is white and have the very best of lighting to work in.
It also depends on the level of methemoglobin.
A good history of recent events must be taken also.
Not everything is as black and white as you make it which is why relying on more than just numbers is a must.