I may have made a mistake............
After undercoating the wedge on the door twice, I left it a couple of days. This was because it was a temporary fix, and I was hoping that the white wedge wouldn't look too wrong against the faded brown door.
I was wrong about that - it seemed to gleam during the day, and glow in the evening. This looked just plain wrong.
On my next foray to Bricomarche, I bought a can of very cheap paint with a dark brown lid. I decided on cheap paint because, remember, this is only a temporary fix. The plan was to add some white undercoat to lighten the brown, and hopefully almost match the colour of the door.
After adding the white paint to a small container into which I had poured the brown paint I was amazed. So amazed that I read the small print on the can.
Welcome to my new, fuchsia coloured wedge. On my next trip to Bricomarche, I may buy a can of brown paint. Until then, I will claim it is a new improved wood primer......
Note from Susan: Oh!...I thought you must have been colour matching to the blind on the door and had done it deliberately!