I recently visited Seville and Granada and it made me wonder if the carvings we see all over the outside and inside of these structures was originally colored or decorated somehow. Here's an example of some carvings I saw on the outside of Granada Cathedral, for example.
Frequently, yes.
Ancient sculptures were almost always painted. Here's an article about how traces of this paint can be detected scientifically: http://www.nature.com/news/2009/090615/full/news.2009.574.html
Here's a reconstruction of what classical sculptures might have looked like originally, when painted: http://www.nbcnews.com/id/11945940/ns/technology_and_science-science/t/scientists-retrace-parthenons-brilliant-hues/
And here's an older article with some history about the rediscovery of paint on ancient sculptures: http://www.jstor.org/stable/30103116
This holds true in the middle ages as well.
For example, the stone crosses you see in parts of England from the anglo-saxon period were entirely painted, and fitted with metal and glass inserts to add sparkle as well as color.
Here's a good article about one such cross: http://www.jstor.org/stable/125057
And here's a reconstruction of the paint on another cross: http://poppy.nsms.ox.ac.uk/woruldhord/contributions/369
Sometimes traces of the medieval paint survive. More frequently, they were cleaned off of medieval buildings and sculptures because, as the paint aged and started to flake, people decided it was unattractive. Sometimes paint was stripped from a decorated building because it was thought to have been a later addition, and the building would look more 'original' without the paint (sometimes, original medieval paint was removed by such well-meaning restorations!).
There are a few interesting restoration projects in the UK right now which are repainting medieval sculptures and buildings their original colors (when evidence survives) - Stirling Castle in Scotland is great example. The result looks very different from the grey middle ages we usually imagine!
