This technique allows you to change the colours of certain parts of the same object and adding a material and texture to it. This is good for materializing facial features like eyes, man made objects and anything you can think of. To change the colours of selected faces only, follow these steps:
1) Select the object in object mode.
2) Add a material to the selected object. In this first material slot you can change the colour of the entire object and give it a distinct name.
3) Add a second slot by clicking on the top "+" icon in the materials panel.
4) Be sure to delete the material in the new slot and add a new material to it with a distinct name.
5) Go into edit mode and select the faces you want to change the colours of. Make sure that you are in the new slot after doing this.
6) Now ad the colour in the new slot. You will notice that the selected faces have not changed colour yet.
7) Finally click "assign" near the top of the materials panel to change the colour on the selected faces.
Here's a tutorial to add materials to an object;
https://www.youtube.com/watch?v=XI-pZshRp8g
https://www.youtube.com/watch?v=wgeP5HKuBX8
