I have been using Marshall's Oils and pencils for a few years now and really like them, but a friend of mine put me on to a much cheaper and easy to use alternative. It's Nicholson's Peerless Water Colors (transparent and self blending) and they're great (not trying to sound like Tony the Tiger). They can be found in most art supply stores, they're made in Rochester New York, and the patent goes from 1902 to 1996 so I assume they've been around for a while. It's a book of colored cards that you cut a small piece from and drop it into water. I use empty 35mm film cans and masking tape to label the different colors. You can also just wet a brush and use the color straight from the card for very vibrant color. The book of colors I have is yellow colored (labelled "Complete Edition") and they are made for films (color slides) and paper. The nice thing about this medium is that it is absorbed even into RC paper where the Marshall's Oils tend to sit on the top and can be scratched if you're not careful.
You do have to practise a bit at building up the colors as you will tend to have over colored your print once it has dried.