Problem importing Blender materials into Unity via FBX

I’ve created a simply low poly house in Blender (2.92) that I have imported into Unity as an FBX. Materials (simple colours) are all set up in Blender.

Initially this was all working fine. Colours all came across correctly (bit dark, but they always seem to do that), but now all of a sudden I am finding that in some cases the material for object A is applied to object B and the material for B applied to A, and so on. Most are fine, but not all (eg the window frames – meant to be white, are coming out dark wood, and the sills – meant to be dark wood, are coming out white.

The only thing that appears to have changed between working OK, and not working, is that I started to apply different colours to different faces of the walls for different rooms. However, the walls themselves are fine, all the right colours in the right places, it is just some other bits as above – and those bits were previously OK.

I’ve read there are issues in importing complex textures from Blender, but these are nothing complicated, just plain colours. Everything looks fine in Blender. What I do before creating the FBX is join most of the objects together in Blender with CTRL-J to minimise the number of objects going across, but again, this was all fine up to now.

I’d be grateful for any suggestions. I don’t really want to have to reapply all material in Unity if I can avoid it.