concerning your question on how to change color of the model, this can be done in general by assigning different material to a model, and in the material you can then select the type of rendering (shader), such as Diffuse, Decal, etc... there you can also specify the specific base color and the textures.
However, this is a basic Unity subject, so I would recommend you search the Unity website on this matter:
Concerning Virtual Buttons:
If you look at the sample, you can see the arrangement of virtual buttons attached under the ImageTarget "Wood".
The ImageTarget also has a component called VirtualButtonEventHandler (you can see it in the inspector view); this component is a C# class that implements the IVirtualButtonEventHandler interface; such interface, in particular has 2 methods:
Those methods are triggered whenever a virtual button is occluded or de-occluded; there, you can put your code to perform specific actions.
For the rest, my suggestion is to explore the sample code (scripts) and scene structure and trying to understand how things are interconnected (as that's on of the goal of our samples !, to teach you about Vuforia ;-)
I hope this helps.