Avgas or aviation gasoline is the fuel commonly used in piston-powered aircraft. There are several grades of avgas including 80/87 (red), 100 (green), 100LL (blue), and 115 (purple).
The most common grade to find today is 100LL (blue). Pronounced "one hundred low-lead," the lead refers to tetraethyllead (TEL). TEL is added to prevent engine knocking. Lead is a highly toxic substance and there is no amount that is not toxic for human beings. Limiting exposure to the blue fuel is a good idea.
The 80/87 (red) fuel is now all but phased out. It was once common but was phased out in the late 20th century due to reduced demand and greater need for financial prudence. This lower-octane fuel was used in low-compression engines. Today many airplanes that were certified for this fuel have special type certification available for the use of mogas, or automobile fuel.
The colors of the various grades of fuel come from dyes which are added to aid in visual identification.
Sumping, or draining a sample of fuel from the aircraft is always on the preflight checklist. This action gives the pilot a chance to verify the quality of the fuel. Do we have the right fuel? Is the fuel free from water and other contaminates? These are questions that the preflight quality check allows us to answer.
Water is heavier than fuel and sinks to the bottom. It is often accompanied by other contaminates. Many sumping tools include a floating ball or other visual clue to help the pilot identify water.