Its not black. Its a very rich and dark brown with just a touch of cherry. If I could compare it to something general - I'd say it is the color of dark chocolate.

27 inches wide and 18 inches tall - and the doors open all the way (hinges are on the outside) so if you have something larger to put in there they wont get in the way.

I bought two for my crop room - teh bottom shelf is at lowest, and the second one is at top so my 12X12 3 ring binder books fit nicely. Unless you want to lose a shelf, only one shelf will hold 12 X 12 from what I see.

Mahogany and Chestnut, I prefer the rich warm color of the mahogany, but everyone has their own taste

You won't be able to use the doors on the top 2 shelves because the way they manufacture it has hinges set up on the bottom shelves to slide the doors into. Those hinges are built into the bottom of the main frame of the bookshelf, so you can't switch that. You could possibly leave the doors off, but you will then see the hinges that I just spoke of and the clasps on the bottom of the one shelf where the doors are supposed to close into. I don't think it would look really bad, but you will see them. I doubt you could take the clasp or hinges off of the other parts without ripping wood off of the product, as it is might marr or rip the wood.