A doctor writes...
it would not be useful for excluding CO poisoning; it is a standard error to avoid relying on O2 saturation monitors if suspected. As pointed out above (and then mysteriously downvoted) the spectra absorbed are identical between oxyhaemoglobin and carboxyhaemoglobin, and very different to deoxyhaemoglobin.
You need a blood gas analyser if you wish to exclude CO poisoning in vivo. Not some fancy colour changing tee-shirt.