Harvey uses extremely thick paint in certain areas because when your light source is from the top, the light hits the blob of paint at a 90 degree angle it will reflect dead on and will produce more light.

When light is hitting at a 45 degree angle you will get some light but you will not get the full effect of the light typical when the paint is applied normally. Harvey will use an Indian Yellow with white in a certain section and within that section he will put a touch of pure, thick white paint.  Behind that section of light, the colors will be more muted, lavender and more blurred. 

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To produce light in a painting you take a little yellow glow make it into a blurry circle then pop some white paint in there.  Note the lavender in this painting and the dark against light.