[Dave Canon]

Dave, I have often encouraged the use of plain backgrounds for that very reason. However, I have found selecting the model (or the background) to change the background has been successful in a few cases but it can be hard work. You do need to learn all the tricks if you are to successfully cut out hair etc. or possibly buy some super software to help do this. An alternative to changing the background entirely is just to change the colour of the background which may be a little easier.

If I have not put you off, I recommend that you stay in colour to do this even if you wish to end up with B&W. You can try making a mask from the subject itself. You may find that one of the RGB channels shows better discrimintion between subject and background. Use that channel as a starting point for the mask and build up the mask by increasing its contrast and filling in as necessary.

One further trick I have found - If the new background has some similarities to the old (e.g. changing one landscape to another which is similar), then you only need to roughly cut round the model but use feathering to make a gentle blend.