Color to black and white conversions are radical transformations of an image. They establish the tonal foundations of a neutral image, creating tonal relationships by determining which areas of an image become light and which are dark. While this process can generate some localized effects (all blues become darker or lighter), this is quite different than selectively lightening and darkening an image to accentuate existing tonal relationships (only select blue areas become darker or lighter). Selectively enhance a tonal structure after conversion, rather than before. Selective enhancement may yield dramatic results.
Here are two ways.