To be frank, I found the original Aztec building textures to be quite awful. This mod will simply replace those yellow and red-brown facades with a desaturated look.
The image to the left is a little crappy but it gives you the idea of what both are going for in general.
To clarify, I did not completely desaturate the textures- some embellishment was left untouched, and you may notice a hint of colour left (lest the buildings look dull).
To install, simply extract the contained 'art' folder into the main game directory.
For steam users, this is '...\Steam\SteamApps\common\Age of Empires 3\bin'
For non-steam users, this will be '...\Microsoft Games\Age of Empires 3'