lets say you want to only colour part of an image, and leave the rest black and white. so that it looks like this:
first, colourize the image and make it black and white.
to do this, go to adjust, hue and saturation, colourize.
if you dont want to do that, then you can just hit Shift L and it will do the same thing.
to make the image black and white, make the saturation 0. it dosent matter what the hue is, as long as saturation is set to 0.
it should look like this:
next, copy [Ctrl C] then undo. [Ctrl Z] then layer. [Ctrl L] now erase whatever part of the image you would like to remain coloured.
now youre done. just rememer to go to Layers, Merge All before doing anything else to it.