I remember those ambiguous missions :D

Before you start the mission, but after you have selected it in the menu, you will see a short description that says:
"The river that runs through the big Forest Clearance creates a stunning rainbow at the waterfall. Take a picture of the rainbow!"

If you want to go straight to the point and don't want to waste your time with riddles this is where it is:
In the small river in the center of the map that flows horizontally. In the screenshot of this guide you can see a place called "Rodungen". The river is just the one below it.
The rainbow is at the beginning of the river (on the left side if you are looking at the map).

I also found you a walkthrough in YouTube where you can see the mission description that I mentioned and the rainbow itself .

Thank you for replying, I cannot complete the mission to take a photograph of the rainbow as I cannot find the motif they talk about, there is not sign of it, can you help please?

No, I got all the achievements when I created this guide and then I stopped playing. But let me know if you need some help

Are you still playing this game?