Hello Priya.
When you say color scalar from an RGB image, does that mean that you want to apply a texture to a model? If so, you can have a look at GPU-based Projective Texture Mapping in a Loadable Module and see if you find some answers there.
If what you really want is applying scalar color values, you can have a look at https://github.com/RafaelPalomar/vtkColorCellsRGBA for an example on mapping RGBA scalars to a model (this is a VTK example, but you can use it in 3D Slicer. To enable the visualization of scalars you can use the Models module as suggested by @adamrankin or programmatically by calling vtkMRMLModelDisplayNode::ScalarVisibilityOn() (http://apidocs.slicer.org/master/classvtkMRMLDisplayNode.html#a9dd055841d24d46216c72f5cce97e6e8) from the vtkMRMLModelDisplayNode corresponding to your vtkMRMLModelNode.
I hope this helps.
Rafael.