How does visible imagery work?
Visible imagery is when the GOES satellite sees returned signals in the visible portion of the electromagnetic spectrum--similar to what would be seen by the naked eye. This explains why these images can only be updated during the daytime hours. Dark areas on the early morning and late evening images are regions where the sun has yet to rise or has already begun to set.

How does infrared work?
Infrared sensors sees radiative temperatures. Cold clouds appear very bright and warmer clouds are much darker. Watch the areas of Baja and Mexico. These areas often turn very dark during the afternoon because the land surfaces heat up rapidly when there is little cloud cover; this is quite common for this region. Conversely, thunderstorm tops are very cold because they are so high in the atmosphere and they will appear as the brightest clouds. In general the higher the cloud is in the atmosphere, the colder it will be and the brighter it will appear.