On infrared imagery, the colours refer to the temperature that the satellite is sensing. Colder temperatures appear as brighter shades of white and warmer temperatures appear as darker shades of grey or even black. Therefore, higher clouds, which contain colder water vapour or even ice, will appear brighter than lower clouds, which contain water vapour at higher temperatures. Often, low clouds will not vary in temperature much from the underlying land so it will be difficult to detect them on infrared images.
On visible images, the colour refers to how much visible light is being reflected. Therefore, the thicker the cloud, the more light is reflected, and the brighter the cloud will appear. In this way, low cloud is more visible than high cloud, because it generally tends to be thicker. Therefore, both visible and infrared imagery should be used together to gain the best snapshot of what is happening at any one time. Visible imagery will not be available at night because there is no visible light at this time.
10:52 EDT Summer doesn't look like it's quite finished yet with a small heat spell on the horizon.