Gradient color of a 3D model

I have a 3D human model

how can i change color of the human model's "ARM ONLY"? 

color keep changing from green to red, and then red to green ( in gradient )

You need to somehow be able to identify where a vertex is located within the body, the simplest method is to use seperate nodes for each of the models parts.

Once you find where the arm is, you can apply vertex color by accessing the color array from a TriMesh (TriMesh.getColorBuffer(0))