This will be gifted with the dishcloth set I made.
I couldn't find a bathmat pattern that wasn't in the shape of something silly. I came across an article about the absorbancy, durability, etc of various cotton yarns and stitch patterns. Using the information in this article (namely, the section titled "Stitch Matters"), I decided to use the "Back Loop Stitch Washcloth" pattern as a base for the bathmat.
I decided that I would make six washcloths (three of each color) and join them to make a rectangle.
However, I have run out of the blue yarn. I'm planning to go back to the store ASAP and see if I can get more of the same dye lot. If not, I'm going to use the natural color cotton yarn I have instead of the blue.
EDIT: I've decided to just use the natural color yarn. It's the same brand as the red (the blue is not), I already have one square made, and it will probably be easier to match her bathroom if it's only got one real color in it.