Ye Olde Chequers Inn

I was going to Photoshop this photo to black & white with just the purple balls in colour. I remember I have to create a layer, cut out the balls, convert to black & white and hey presto its done because the colour in the underneath layer shows through.  However I can't remember how to cut out the balls or convert to B&W. I did a Photoshop course last summer and have now forgotten it all because I have not used it. Can anyone remind me?

More details on this building are contained in a previous post if you are interested in history.