To maximize this clever hack, use the lightbulb before laying down your primer. This step will allow you to circle any blemishes or problem areas with a pencil.
Using a lightbulb also helps determine what a paint job will look like under artificial light. Consider the Color Rendering Index (or CRI) of your lightbulb to aid you as well.
Light sources usually range from 70 to 100 CRI, 100 being the optimum given by full natural light. A lower CRI makes paint colors look more drab or grayish than full natural light.
By testing out bulb differences during the painting process, you can see the benefits of higher CRI bulbs and evaluate whether or not the paint color is exactly what you want.