This is the only kind of easel I have, and I like them very much.
Mine are made from black core double mat board. The only thing I don't like is that if I'm very close to printing the full frame, it can be hard to see if the edges are all excluded because the image is harder to see on black. It's not really a big problem, you just have to be careful.
If the bottom mat board is a non-actinic color such as orange, the image will still be plainly visible. There should be no light reflected back from the bottom board to degrade the image. Some older easels were white, and this could cause some fogging in the shadows, especially with thin paper.
You can find Rubylith on ebay. But, since you're in Sonoma, you can also get it at the Art Blick store in Oakland. They carry it because a nearby art class uses it.
Perfect, thank you! I work in Oakland and am down that way about once a week!
And thanks Jim that's a good idea, I'll keep my eyes open for orange or yellow. I cut the window with a mat cutter and it's the edge of the image on the bevel that would be most helpful to see, so in my case it will be the top mat. ( Not sure my post was clear... I've been using black-core flat black mat board. )