Texture and primitive color

Hello forum,

I am mixing texture and primitive color. Check the following image:

http://imgur.com/dpLE94s

I want to add color only to the 4 corners of the primitive so that fragment shader can interpolate in between.

Now i am using ofPlanePrimitive object and i found that 100 vertices using the following sniipet:

//--------------------------------------------------------------
void ofApp::setup()
{
   if(ofIsGLProgrammableRenderer())
   {
      shader.load("shader/shader-texture");
   }
   else
   {
      std::cout << "Programmable shader is not supported." << std::endl;
   }

   //load the image
   image.loadImage("puppy.jpg");

   plane.set(800,600,10,10);

   plane.mapTexCoords(0,0,image.getWidth(),image.getHeight());

   //add colors to the plane primitive
   ofMesh *mesh = plane.getMeshPtr();

   //get the size of the vertices inside the plane primitives
   unsigned int sizeVertices = mesh->getVertices().size();

   for(unsigned int i=0; i < sizeVertices;++i)
   {
      mesh->addColor(ofFloatColor(ofRandomuf(),
				  ofRandomuf(),
				  ofRandomuf()));
   }   
}

Any hint to get only the 4 corners of the plane primitive instead of the 100 points which i am getting now?