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Subjects without much color are a great way to focus on value. I literally made a little chart on my palette as I painted that went from 1 (the lightest - in this case the highlights) to 6 (the darkest darks). Then I just squinted and compared values to see where they repeated themselves. For example the shadow value between the florescent tubes (I called this value 4) repeats in the shadow cast by the incandescent bulb. Once I had the masses blocked in, I could go over them and make small adjustments for subtlety.