I'm preparing the demonstration for our class tomorrow in which I show how to determine the values of the colors, grouped into masses. I'm using Photoshop Elements 2.0, which has a neat feature. You can POSTERIZE the image.
To do it use
>Image >Adjustments >Posterize
I find that 5 values works best in most instances... It's a great way to start thinking values of colors.
I'll show you in class tomorrow. If you want to bring your laptop, that might be a good way to go.
For the rest of you following along (lurking), I suggest you experiment with Photoshop and see what it can do!