Hello,
You can use whatever fabric you choose. The 16ct. black Aida is just want the designer recommends for the design. If you are going to switch to a 14ct. Aida then you will need a fabric size 16 x 18. We recommend at least 2 inches of extra fabric surrounding the design. A piece of 16"x18" fabric will give you 2.2" of fabric on the left and right, and 2.3" on the top and bottom. The stitched area will be 13.7" x 11.4".
Personally, I have not stitched anything on a dark colored fabric. I stick with the lighter colors because it's easier for me but there are people who have stitched on black and navy colored fabrics that post on here. Hopefully, they can help you decide which one is best to go with :)
~Kristi @ ECS