The site would make the decision for you if you are not using Green Organic they even check my caravan before i was allowed on to my service pitch
No, if all they care about is the colour of the fluid, rather than whether it's harmful to the environment, then I would make the decision between buying a different chemical just for its colour, or cancelling my booking and going somewhere else. It would depend how much I wanted to stay on that site - but it wouldn't get as far as the site making the decision for me, because if I'd decided not to comply with their requirements then I wouldn't be going to that site.
In my working life, if someone specifies a requirement which is quite clearly based on a lack of understanding of what they're asking for (e.g. believing that the colour of loo chemical is more important than what's actually in it) then I'll quite happily debate the matter with them, but caravanning is my hobby and my leisure time is too short to argue over something that can be resolved by voting with my feet.
Having said that, I would be tempted to do an experiment with my current, environment-friendly but wrongly-coloured, chemical; If the blue colour only comes from a dye rather than an actual chemical constituent, then straining a sample through cat litter (or any other Fullers Earth product) should strain out the dye, then I could just chuck in some green food colouring.
Note that I'm NOT saying I'd use this technique to enable me to use environmentally-damaging product against the site's rules, because I wouldn't: But if the only difference between what I've got, and what the site want, is the colour of the dye, then I'd consider changing the colour of the dye was fair enough.
Just out of interest, are they equally specific about what washing up liquids or soaps you can use?