Well, in that example it would be easy enough to select just their shirts and the one pair of shorts.
- I recommend masking and not just using a selection tool like the lasso etc...
- Once you have fine-tuned your mask to contain only what you want, save it as a separate channel just so you have it for later use. It'll will make life easier.
- Now exit mask mode and then choose to select inverse.
- Now everything BUT the shirts is selected.
- Make all that black and white (and there could be a whole lecture on how to best make it look like a black and white photo, but just do what you want even if it means simply removing all the saturation - but in that case then at least do it as an adjustment layer so that you can turn it on and off and adjust - instead of altering the actual layer of the photograph).
- Deselect everything.
- Go to the channel where you saved your selection after masking and load that selection so that the shirts are once again selected.
- Make whatever color changes or adjustments to the shirts to fit your need.
I also recommend creating snapshots
(right-click on history panel - then choose new snapshot) of different possible final versions, so that later you can quickly go to different snapshots so compare the possible final versions.
I don't know how well you know photoshop. If you don't know how to mask or save selections or use adjustment layers or snapshots, then search the internet for tutorials or you can ask me a question (as long as it is not something that would take a while to explain, I'll he happy to help - and not just me - I'm sure others here would as well).
P.S. - the key to this looking good depends on how well you mask.
Also, if you are left with hair in the selection that remains colored, you can later go and use the sponge tool to carefully desaturate only the hair strands.
These links might help:
http://www.photoshopsupport.com/tutorials.html
http://www.photoshopcafe.com/tutorials.htm
