What color is 100LL and why do we use it? 100LL contains tetraethyl lead while you use unleaded fuel in your car. Why? Lead prevents detonation and an applicant for a pilots certificate (license) should have some knowledge of what detonation is and why we are concerned about it. 100LL is blue and Jet Fuel (commonly Jet A) is the color of straw. I’m surprised that many folks think the “Jet Fuel” is some super high octane fuel. Jet fuel is basically kerosene and can be used in diesel engines. If you put jet fuel in your piston engine airplane the engine will not run for long and this contamination has caused many serious accidents. So your fuel is blue to help you determine if it has been contaminated, especially by jet fuel. And the subject of fuel leads to engines. You are not required to know how to build a piston engine for a private pilot check ride but you should have an idea of how it works, what makes it run, and what makes it not run.