Hi James, here is one way that I have learned, though it is certainly not the only way or perhaps even the best. I am still a relatively new user myself. However these would be the steps I would take to quickly get desired effect.
This assumes you would not be using body paint and uv editing.
1. Create your cube
2. Make editable by pressing C
3. Select the use polygon tool (a fast way to scroll through until your at surfaces is by pressing enter)
4. create a new material and choose a color
5. repeat step 4 only with a new material and color
6. select a single surface of the cube and while it is highlighted drop on the cube
7. select another surface and use another material.
You can do this to as many separate selected faces as you like on any polygonal object. Have fun