Another technique for adding colors to your model is to unwrap your object to make a flat 2D representation of the surfaces, which is called a UV Map. That 2D image can be colored and decorated. Your colors would not be limited to specific faces. UV Maps give you the ability to add very specific details and shading to your object. You can color the image inside Blender with a process called texture painting. You can also work with an image editing software you are already familiar and comfortable with. In either case, the very first step would be making the UV Map to color.
Coloring with UV Maps
Adding a new panel to Blender
When working with UV Maps, it is helpful to add another panel to Blender. This will give you visibility...