Consider a portrait where a person has no fashion sense or maybe Scottish and is wearing a red and blue plaid shirt. If the blue is near the same intensity as the red the checker pattern will blend in when you convert to B&W. Well the quick answer is to use either the channel mixer or a selected colour filter when you do the conversion this would either selectivity darken the red or the blue. But wait although the plaid shirt looks OK the skin starts to look blotchy, not very flattering for a portrait..
The answer is rather simple you selectively blend 2 B&W versions of the image together. One version optimizes the plaid while the other optimizes the skin. If needed you could add more versions to optimism additional parts of the image.
|Original Colour Image|
In this example I used the channel mixer to create 2 images plus a 3rd which is the luminance channel from the original colour image.
|Left: Red Berries Centre: Green Background Right: luminance channel|
|Final B&W with selective contrast filtering|