Sometimes Veggie Oil based fuels or blends of petroleum and veg. oil can appear reddish in color in the prefilter.

But, yes it is most likely dyed fuel. I would never do such a thing in my car, but my friend's prefilter looked just like that when he was using dyed fuel.

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83' 240D 204,000 miles SVO conversion - Heated Pickup Stock Tank, 3 Gallon purge fuel tank, HOH, Coolant heated filter, FPHE, 2 Greasecar Valves. 12V pump for Veg. and stock pump for purge fuel. Injector overflow return to veg. tank.

98' VW TDI Jetta SVO Conversion 154,000 miles/57,000 WVO miles