It used to be simple when we had a hot tap and a cold tap!
Nowadays there are mixer taps, and so you have to know which way is hot and which is cold. So they use red for hot and blue for cold. Some of my taps at home have no markings do you just have to guess.
The classic version is more like this :-
Now, to my mind, turning the tap so that the red dot is aligned with the water spout should be hot, and with the blue dot aligned with the water spout it should be cold. So this would be hot :-
I am wrong, of course, and what it means is turn tap towards the blue dot for cold (anti clockwise) and towards the red dot (clockwise) for hot. What is confusing is the dots are being moved so you are not really turning the tap towards the dot, you are turning the dots too. If the dots were on some part that did not move, that would work. If the dots were arrows, or even triangles indicating an arrow head, I would cope much better. But I am learning that this seems to be the convention... If only Steve Jobs had designed taps!
But when it comes to something like this, I hope you agree it is different :-
My shower at home is very similar to this with temperatures marked and a button next to 38 degrees which you have to press to get hotter. It has a mark on the base and you turn the knob to align the required temperature with the mark.
The one pictured, I assumed, was the same, but instead of specific temperatures above and below the 40 degrees marked, there are red and blue marks which I assumed meant hotter and colder.
So I assumed that if you you turn so the blue markings are aligned to the notch on the back, that should be colder. Similarly if you put the red markings next to the notch on the back, it would be hotter. This is obvious, surely.
But no! This is like the tap, though in reverse, in that turning anti clockwise gets hotter not colder. Even though the notch matters in that you align the "40" with it to get 40 degrees, the red and blue are not indicating where on the dial is hotter or colder, but a direction to turn the dial.
And just in case you think it was plumbed backwards, no, the button stops you turning it as shown above. I.e. you have to press in order to align the blue marks with the notch to get hot water.
They fix this with a sign saying "Please check the water temperature", which is clearly an admission of a non intuitive interface to a shower...
(this was the shower in the cabin on the cruise).