I give the knight attacking Becket a simple chain-mail tunic and helmet. He’s placed in the cast shadow too, so he looks darker except for the light reflected from the floor behind him and the blade of the sword that’s in the stained glass light. This makes the knight seem more menacing.
I give a touch of colour to the knight in the background and we’re almost done. I add the beams of light coming from the windows and I stress the luminosity with an additional Color Dodge layer. Finally, I paint small dabs of paint in a very light blue, which simulate the effect of dust hit by the light.