In the USA, most ATF comes with a dye that precludes use in "on-road" vehicles. If a tank gets dipped and it comes up red, you can assume the position for the tax man, even if it was an aftermarket additive to taxed fuel. For a generator it's not an issue - those can run on untaxed (that is "road taxed") fuel with no problems, and you can have 5-gallon cans of red dyed fuel so long as it doesn't end up in your street driven vehicles.
For a long term stored generator running on diesel, the best advice is to run it monthly (at least) to get the engine up to temperature so that the oil will create a clean film on all the contact surfaces and will evaporate any accumulated moisture out the crank-case vent. Use some form of fuel stabilizer, as the newer lighter crude diesels that have been produced in the last 15+ years are not as stable as the older fuels. Other than that, keep your fuel tank full to reduce the amount of air in the tank - air in the tank will bring with it moisture, and tank walls that cool down will cause condensation on the walls which will settle to the bottom of the tank. Algae will begin to eat diesel fuel at the fuel water boundary, and then will die once satisfied and settle at the bottom of your tank as sludge. This sludge will gum up filters and depending on the quality/condition of your filter could get further into your fuel system.
To keep my 40' Transit Bus stored for the last 13 years (prepped for deep storage), I've been running Pri-D and Pi-Ocide (man, you have to get a 5-gallon pail for this stuff though). Pri-Ocide is an anti-microbial/inhibitor (algae). Once a year I polish the fuel by running it through an external marine fuel filter and a 12V transfer pump for three hours. The filter is a water separating type (Racor), and is rated at 5 microns. Three years ago I ran the engine for a ten-year test, and it fired right up with no unusual smoke or sounds (I say unusual because it's a Detroit 6V92-TA, they smoke at idle or loaded - they also have a saying, if i'ts not leaking it's out of oil...).