Red Diesel
As I understand it, red diesel, can only be used legally in vehicles that are classified as agrcultural vehicles, eg tractors & combine-harvesters. Many farmers keep tanks of red diesel on thier farms, for this purpose. However, it is illegal for such farmers to use the red diesel in any other non-agrcultural vehicles they own, such as a car or lorry, or even for the farm house central heating boiler.
The red dye in the duty-free diesel, is designed to stain the engine on a fairly permenant basis. I understand that the usual method that C & E use on spot checks of suspect vehicles, is to remove the fuel injectors, as these will be red.
So, I would imagine that it would be illegal to run an aircraft on red diesel, even if this was a suitable fuel. All the so-called "diesel" aircraft piston engines that I have heard about, are designed to run on Jet A1.