On Sat, 06 Mar 2021 21:17:32 GMT, JohnP wrote:

I want to have a different coloured wall when I decorate the lounge.

20 x 12 feet

window & patio on one long side
fireplace & TV on one short side
doorway to hall other long side

I feel inclined to have the contrast and slightly darker wall as the long
side facing the window.

Are there any conventions or effects that this goes against?

I'd do the wall that you look at the most - the one with the fireplace
and TV. (Against that, when I had my kitchen painted a few years ago I
had the wall opposite the window painted a different colour, as two of
the other walls were lined with cabinets and the third had the window,
there wasn't a lot of choice, but I though, and still think, that it
was better than having it all the same colour.)