Every body refers to Merida in Yucatan as the White City, but once you stroll around there is no general tendency of having white buildings, not even downtown which is the oldest part of town, then, Why is the nickname so encrusted in the city's tradition?
When the Spanish arrived to the area, there was a huge Mayan city called Tho, most of the buildings and pyramids were extremely white (I think to fight the extreme heat in the region) and the conquerors nicknamed it the white city, and the name got stuck forever.
The pyramids were so massive that the last basements were finally destroyed at the first part of the 20th century, more than 300 years after the conquerors began to destroy all signs of the Mayan religion and culture.
What you find in Merida is a very European style of construction, due to the tight ties with Paris and Madrid the rulers of the region had. If you really want to see Mayan houses, you need to travel to the outskirts of the city, after many centuries, the people in small towns still build their houses the Maya style, an oval structure with thatched roof, that happens to be very economical, cool during the warm months, relatively easy to build and using only local materials.
Here is an interesting link to the Maya House