In the middle of a face is an average color of all the vertices. For triangle this means:
    
Col_RGB = ((R<sub>1</sub>+R<sub>2</sub>+R<sub>3</sub>)/3, (G<sub>1</sub>+G<sub>2</sub>+G<sub>3</sub>)/3, (B<sub>1</sub>+B<sub>2</sub>+B<sub>3</sub>)/3)

Where each vert has color in form of (R<sub>n</sub>, G<sub>n</sub>, B<sub>n</sub>) for n in (1..3).