These are the rules on this site
Customers are only permitted Green Organic Environmentally Friendly Toilet Fluids
We no longer permit the use ,or on-site disposal ,of Pink & toxic formaldehyde -based blue chemicals
This is exactly why I said that if I was that interested in staying on that site, I'd call them in advance to check exactly what their requirements were. Because based on the second line of the rule, it seems like my formaldehyde-free blue chemical would be accepted, and that the "Green" in the first line refers to the environmental credentials of the product rather than its colour.
But as the site have specified what they allow, using one word ("Green") that has more than one meaning in common use, and another word ("organic") that has no legally-supportable definition in this context, only the site can tell us what they really mean.
Either way, the new information - that my caravan is going to be searched, and the contents of my bottom tank tested, on arrival - means I'm not going there, whatever the rules.