Each grade of AVGAS is dyed a different colour to make it easier to identify:

Aviation fuels are colour-coded to allow quick visual identification and to prevent the catastrophic consequences of misfuelling, particularly the accidental use of Jet A-1 (turbine fuel) in a piston engine aircraft or AVGAS in a turbine engine. 

AVGAS 100LL (100 Low Lead) is the current standard piston aircraft fuel and is dyed BLUE. The LL designation refers to the reduced tetraethyl lead content compared to older AVGAS grades.  The colour coding system is internationally standardised and is a critical pre-flight fuel check item. Pilots must verify both the fuel type label on the refuelling equipment and the colour of the fuel on the stick or draining into a fuel tester. 

Blue colour → CORRECT. 100LL AVGAS is dyed blue as the international standard colour for this fuel grade. The blue dye is added to the clear base fuel during production and serves no purpose other than identification. A blue fuel sample in the fuel tester confirms 100LL AVGAS.

Red colour → INCORRECT. Red colouration is not a current standard AVGAS grade colour. Some older AVGAS grades in certain countries used red, but 100LL is universally blue. Red fuel would indicate an incorrect or non-standard fuel and should prompt immediate investigation.

Yellow colour → INCORRECT. Yellow is not a standard AVGAS colour. Jet A-1 can appear straw-coloured to pale yellow in some conditions, but this would be contaminated or degraded Jet A-1, not AVGAS. Pure Jet A-1 is colourless to very light straw.

Green colour → INCORRECT. Green was the colour of the older AVGAS 100/115 grade.