[quote=aaronf]...i was wondering if it was possible to spray a working fixative over the whole thing and start layering in color once the values have been firmed up?
im asking this because i would like to keep the graphite as an underdrawing.../QUOTE]
The short answer is, Yes, it is possible to layer colored pencil over a graphite under drawing without damaging or destroying the graphite under drawing. However...
You will want to use something more permanent that workable fixative. Workable fixative allows you to change whatever was already on the paper and you don't want to do that.
You can start color work with a earth tone under drawing, but you'll still need the fixative to protect the graphite under drawing.
The better course of action in my opinion would be to begin again and start with an umber under drawing. Develop that just as you would a graphite drawing, then layer color over it.
How much you develop the umber under drawing is entirely up to you. Here's how I do it