I love prismacolor, but it is hard to explain why without you seeing the effect in person.
I suppose the best way of describing it is that you get a much better result of light and shade with prismacolor than any other medium such as h2os or watercolor crayons because you draw color in with the gamsol and so you end up with dark edges and it gradually gets lighter all the way in to the centre of the image.
I find with watercolor pencils and h2os which I have both that you are basic coloring in an image and it is far more difficult to make it look like an image would look in real life, there is very little texture or depth to the coloring.
Having said all of that, I still think you need as many coloring options as possible to get the maximum potential from each stamp set.
I will now get off my soap box!!!
To Prismacolor or not to prismacolor?
