Quick Mask Mode. This will allow you to take the paint brush tool and paint around the object you want to select, then convert it to a Clipping Path, which will allow you to do anything you want to just that selected area.
yep, the easiest way to do it does depend on what you want to colour etc though. Magnetic lasso is good for some things, selective colour is good for others, it really is a case of try it and see for the first few times in my opinion.
I normally copy the 'colour' bit to a new layer and duplicate the original image so that the original is always there until you know you've finished. The b&w can be done via desaturate or one of the layer filters.