The more I see of these, the more I think this was a regulatory issue.
The new ones use the federal highway standard typefaces and route shields, and on western way all the wayside signage (things like the yellow diamonds that warn you of hazards) were replaced with the standard versions.
I don't think the purple was the problem - all the other weird choices were.
They could have made these purple, but I think if they were going to replace all of them for compliance reasons they decided to just update to a more modern scheme.