I've paraphrased Red Green before in relation to music. In this case, it's a direct quote.
"My definition of art is, if I can do it, it's not art" - Red Green
I'm pretty sure I can paint a rectangle a solid color--very sure, actually. So, by this common-sense, common-man lithmus test, the solid color fails.
Edit: Come to think of it, even a monkey can do this. Just give him a canvas, a brush, a bucket of paint, and whatever time it takes. Everything else in the room may end up one solid color too, but that doesn't matter right now.
Edit 2: You realize, of course, that if a canvas painted a solid color is art, then every blank canvas sold is also art?