Your wife is gonna kill you telling the word how old she is
So, before you die we had better help you a bit
Im not too sure pastel will give you the effect you are after, I think it will just overwhelm the graphite drawing, if not just totally cover it.
I think going lightly over with a pencil crayon will be your best approach. Put down some rough tone on a piece of paper and go over with pastel and the coloured pencil and see which looks the best.
Best to experiment first than totally mess up.
Phil
www.graphite-art.com