Some people find it very difficult to follow when printing is on coloured paper.
A friend’s daughter who is dyslexic had to sit an exam paper printed on light blue paper.
Judging by your lovely comments about my trough, looks like I will be buying paint, just wish it wasn’t so expensive, nearly 17 euros for 1/2 Ltr
Leroy Merlin own brand is very good (Luxens) buy a big pot of white and mix up some colours into smaller pots. Action have cheaper paints but I don’t know what the quality is like.
I enjoy mixing colours, my lounge looks like an off white but during the day when the sun moves around it subtly changes colour. I note in what proportions I have mixed in case I have to do touch ups later.
Sadly those stores are not in my area, so it has to be Bricomarche 
Blue is not the only colour used. Different people use different colours to help them see the print clearly. When I was teaching we used a multitude of things to help students with dyslexia manage their learning, one of the tools was a set of coloured acetate overlays which the student would place over the white paper, they used whichever colour helped them and they could carry it with them without the need for coloured paper. It also meant they could read standard books.
Obviously this didn’t work for all students as each person is different but it’s a great tool for some.
Love the troughs!
You could always use linseed oil to treat them instead of paint, although that isn’t a cheap option either. We use it on our shed, troughs and furniture.
I treated a wooden fence with some rather fetching (IMO) blue-tinted lazure from the now partly defunct Weldom (in some cases now Bricomarche.)
Lasted years, & resisted the sometimes very inclement weather, very well.
I’ll probably get the same colour green so they all match. I have put some gutting under the eave of the shed where the other big one is and after all the rain, my water butt (hidden behind the shed), is full up
We used a rather expensive paint from UK for our shutters. Although it was expensive it has lasted brilliantly and after 9 years looks almost as good as when we did it. It was Sadolin Superdec. I am not sure if it is available in France but well worth the cost as it lasts better than anything else we have used. Water based and needs no undercoat.
This link might help
Never had a problem with paint here, except it’s rather expensive