The purple way finding signage that you now see across the resort, while designed in the late 80s, still holds up quite well in my opinion. It certainly helps distinguish when you are or aren't on Disney property. And for the most part RCID (is that who actually oversees signage?) stays on top of it and updates it as needed.
Plus, as
@WhatJaneSays noted, quite a bit of thought went into the final product. Sussman-Prejza were the design consultants. The color scheme was intended to be unique and pay homage to Mickey Mouse, and a whole hierarchy was developed to help guide guests across (what was then) a rapidly expanding resort. The whole package was designed to be memorable, but also legible and practical. And I think they pulled it off rather well...
http://www.sussmanprejza.com/sites/default/files/Disney Euro.pdf
http://articles.orlandosentinel.com...9002183365_1_trendy-colors-mickey-mouse-signs
So then next time you're on property, remember that the road signs weren't done on a whim or only to be cute. A lot of effort went into them, and the fact that the same basic guidelines are still used nearly 30 years later is testament to how well they were designed.