[daeken]

Simple: You mix the colors one way, paint for a while, go off to some other part, mix the first color again, and paint again. If you can't tell the differences between two colors, it's easy to come up with two similar/'identical' colors that are really quite different to someone with a different color perception.

While this article doesn't remotely prove that this is what's happened, it seems like a possibility to me.

[wolfhumble]

The effect where colors with different spectral power distributions matches under certain conditions due to the observer or the light source, is called metamerism.

http://en.wikipedia.org/wiki/Metamerism_%28color%29