Here's my high contrast b&w/color secret grain technique, This workflow mimics making traditional prints from a film negative. This is quite different from just adding some grain on top of a photo afterwards.
convert the raw file to a very flat file, no contrast enchangement or added saturation in the raw conversion.
bring the converted file to photoshop.
add a new empty layer on top of the original, fill the empty layer with 50% gray. Add noise (3%, gaussian, monochomatic), and change layer blend mode to linear light.
add the big contrast/levels adjustment layers on top of the noise layer. You can make quite strong contrast moves, because the noise layer will smooth any banding of value in the original file.
adjust the amount of grain with changing the opacity of the noise layer. maybe add a small gaussian blur (0.2 or 0.3) to the noise layer if the grain feels too sharp.
I'm using a more refined version of this technique, to control the grain better, but the idea is the same...