As a colorblind guy, I see a lot of Ishihara dot tests. This painting in an alley in Fort Collins looks like one to me.
Colorblind photographer you say? Yep.
UPDATE: A fellow photographer in Fort Collins just let me know that the power transformer that this painting is on was painted by a local artist, Amelia Caruso. While her main web site seems to be down, I did find this web site where you can see her other work. Amelia, if you see this blog, nice work!