This is my first tut and haven't seen this here, or at least hope not.
this is how to use the channel mixer to make a image b&w and colorize an object back.
original photo
to this
first, open a image and go to layer>new layer adjustment>channel mixer.
then click monochrome.

now that it is b&w, make sure your brushes are black and white by default, then select black brush to put color back in, if you do too much, use white to fix.

added some text and finished.

easy tut for beginners like myself.