You will frequently see references to objects having been photographed or observed in "red" light, "blue" light, etc. This refers to the fact that a colored filter was used to accentuate certain features. With a red filter, for example, the red end of the spectrum is passed to your eye, while other colors are blocked. When observing Mars, this makes the planet appear bright red, and by blocking other colors such as contained in the surface markings, these features are accentuated because they appear darker. In this case, you would be observing in "red light". Even a black-and-white photograph is said to be a red light image if it was shot through a red filter. "White light" usually refers to the fact that no colored filtration is being used at all. The main exception is in the case of the sun. We observe the sun as a projected image or use a filter to darken the sun to the point that it is observable. Depending on the material used, a filtered solar image will appear either blue or orange colored. Despite that, we are still said to be observing the sun in "white light" because the colors of these filters do not substantially affect the visibility of features on the sun. If you were using a hydrogen alpha filter to observe solar prominences, then you would no longer be observing in white light, but in hα light.