Well, they didn't notice, even with having our company meeting today. Of course, this isn't all that odd given that, with one person out on vacation, the two bosses not in on the prank, and two employees not local, there were only six of us in red shirts. Since one of the employees leaves around 2:30, I decided we needed to do a "reveal" while we were all still here. The bosses seemed to appreciate it. One of them had noticed that the married couple had the same color of shirt on, but otherwise didn't notice anything.
It was pretty fun, although I found out on Friday that one of the employees chafes at any sort of group activity like this, although she was willing to play along. So, I think that means I won't be doing something like this again.