How are SLS printers able to print multiple colours?
The Z-corp/3D systems printers lay down what is essentially ink in each layer (only around the perimeters) much like an inkjet printer, dying the powder as the parts are made. This means they can make almost any color at any point in the model. The down size is these models are pretty fragile, at least the last ones that I have handled. This can be helped by dipping them in cyanoacrylate and letting them dry. 

As pointed out in the comments, this is not an SLS process, but looks very similar. The printer lays down a binder (clear or colored) on each layer, and is why these models are much more fragile than SLS models, which are very strong.

Here is an example of some prints: http://mcad3dprintingandprototyping.blogspot.com/