Painting a through-room

[LordOfTheFlies]

I've got a back-room and kitchen/dining room extension ( L shaped).
Currantly all room painted 'Biscotti' beige-y.
The back room always seems a bit dark so I want to paint it, pale lemony/cream but not the kitchen/dining room.
There's a squared off arch (which was the original back wall so about 12" thick) between the back room/dining room.
Where do I stop the pale lemon. Just on the wall or on the onside of the arch?
TIA

dont stop,just keep going

But I don't want a lemon kitchen ,my tiles are cream,biscuit,brown sugar and terracotta!!.
There has to be a cut off point

well cut at the end of the arch
or have the arch in a different colour?
(we only have one colour in our house as I am C Blind!)

I think you need to paint under the arch in the lighter colour. But are you sure it wouldn't be better to paint the back room in a lighter shade of biscotti?