A fluid testing system comprises a source that generates electromagnetic waves, a detector that receives the transmitted electromagnetic waves and generates analog signals corresponding to the colours represented in the electromagnetic waves and, a receptacle, having a fluid inlet and an optical inner tube with transparent walls. The receptacle is positioned between the source and the detector to enable the electromagnetic waves to pass through its walls and through a fluid sample in the receptacle. A repository stores a pre-determined range of reference values corresponding to the values of digital signals for fluids of various colours. An analog to digital converter in the system cooperates with the detector to receive the analog signals, converting them into digital signals, wherein the values are compared with the reference values by a comparator. A fluid outlet provides tested fluid. Further, a fuel supply system is disclosed for supplying fuel to a vehicle.