Single extruder
*You can color the filament as it is pulled with a sharpie marker either by hand, or there
are a few files on Thingerse that will color the filament for you.
*There are a few, though fairly hard to find, filaments out there that make a near tie-dye
looking filament. But this is does not give you the ability to control which color extrudes where.
*Many people have experimented with multi/dual colored filament using a Filastruder.
Dual extruder
*Obviously gives you two options for colors. But if you use a sharpie or before mentioned
multi-colored filaments or sharpie technique, you could easily have 4 colors. But again, it
is either tricky or impossible to control the extrusion of each color.
The Kraken
*The Kraken is a quad extruder. This obviously allows up to 4 different filaments to be extruded.
You can read more about the Kraken at Filastruder.com or at e3d-online.com
Hope this answers a few of your questions 