But what is the blue one for?

On an allotment in the forest of the neighbouring village, a big allotment with cattle (bull, cows, calves), I saw these portable toilets.
In Germany we are meanwhile used to waste sorting. And to the different colours of the waste bins (or at least of the lids).
Normally you see one portable toilet, or if multiple, they are all of the same colour.
But here we have four different colours. Looking for the analogy with the waste sorting I tried to think what colour would be used for what kind of ...
The yellow one is clear, we all know the yellow liquid we produce, starts with p.
Then the brown one? Okay, we produce also something brown, starting with a p too.
Now it became tricky. I assumed the red one was for ladies only.
But what is the blue one for? For the lords and dames?
Any ridicule hint is welcome.

Thanks to laurie54 for hosting the Silly Saturday "challenge" this month.