Dark brown leather

I would get a neutral couch. You're going to invest a lot more in it and it will be with you a lot longer than a cute little accent chair.

I would use a neutral. Medium grey would work.

Try a search of living room photos using the term Dark Grey Walls. You could use any of the colors in your chair for your couch but I'd have a vision for the whole room, rug colors, pillows, window coverings as well. Cute chair!

You could pick out any of the colours in the accent chair (except the chartreuse, although you can pick that up in accent pillows).

A block colour - neutral is good, also navy or red. Don't go dark grey, although light grey could work.

I would go for a light to medium grey couch

Sorry missed dark brown walls I switch to off white or khaki couch

go for white, off white or cream colour. Additionally you can buy green, rust cushions to add colour

Do you have rug or carpeting you are working with? If not, and you plan to get a rug, I recommend you do that first. You are kind of starting at an interesting point! Do you have pets or children? These are factors in this decision, so please give us a bit more info so we can give better advise.

Dark gray walls and light gray furniture always works. I think something neutral would be best, and let the chair stand on its own as the main accent. Bring out some of the brighter colors on that chair with pillows, throws, or other small items.

Might be hard to find but I can picture a sofa in the earthy rusty red dominant in the accent chair. It could probably withstand wear and tear better than a lighter neutral. Use a neutral but textured rug, neutral walls and have fun adding throw pillows to pick up accent colors.

I don't want to sound harsh or rude, but basing a design plan and color scheme on an armless accent chair, meant for a bedroom or very occasional use, is not the long term way to choose a sofa.
Start with your expensive pieces that are the workhorses of the room, the sofa, rug and armchairs. Accessorize later.