The type of fuel a bush plane uses depends on the type of engine it is using.
The most common type of engine for most small aircraft and general aviation is the piston engine, and this is, therefore, the most common choice among bush planes. Think of these engines as essentially similar to a car engine, only with far stricter tolerances and performance qualities among some other differences.
The reason piston engines are so popular is that they are cheaper to build and maintain than turboprops, and can also be started without the assistance of ground facilities or attendants.
Most piston engines in bush planes will use avgas, also known as aviation gas. However, there are a few different variants of avgas available.
Avgas 100 is the main high-octane fuel used in these types of engines, which has a high lead content and is dyed green.
The other fairly common avgas is 100LL, which is the low lead version of avgas and is colored blue. The different coloration of these fuels makes identification quick and easy.
While there are a few other types of avgas, most are rare or long out of production.
It is also worth mentioning that some bush pilots in very remote locations can find avgas difficult to obtain, and so these pilots may well use turboprop engines that can burn other fuels, such as kerosene-based jet fuel.