I like the slate blue of the corded tie back, but it might be a little dark for a north facing room, however, all could be corrected with nice lamps and shades with warm white bulbs. If blue is not to your taste ( personally love a good blue ), then go neutral with the same buff / stone colour in the curtain.

You could go for completely white washable Matt, then around each boys bed you could have objects on shelves, rugs etc that are particular to them and their ages. This will help you create a room applicable to both boys. Plain white will also make the room look larger. You could also go for a pale light blue or a light grey as a base and do the same with shelving for night lights etc.

I wonder if you have space for something like this on the wall between kitchen and window. https://www.ikea.com/gb/en/products/storage-furniture/shelving-units-systems/veber%C3%B6d-shelving-unit-natural-art-10343328/ Scroll through the pics on that page to see them with stuff in them, so you can get an idea of how they would work for you. Maybe two of those? It will give you lots of storage and won't block the ventilation (hopefully). You can clamp a light on it, on the kitchen side, to give you light above the counter. With the central light you'll be casting a shadow on the counter/cooktop, so you'll be working in your own shadow. It's just an idea :-)

Here are the paint colours in that picture. How dark a colour is does depend on the light. For example I painted my hall, stairs and landing in Pigeon by F&B but because the area has little light it has come out really dark! A lighter green is Mizzle or Cromarty. I have Cromarty in my living room (2nd pic).