I wouldn't be too sure. If someone sees someone filling-up at the Red diesel pump all they need to do is take-down the license plate number and report it -- most States are VERY picky nowadays when it comes to tax evasion. And my understanding is that the red dye stays "in the system" in detectable amounts for quite a few tankfuls after even one use.

I wouldn't think the tax evasion would be worth the risk myself. If one wishes to cheat on their taxes, it's probably safer to limit it to your income tax forms.