You can work with your currant wall color or go more yellow - that is up to you. If you do decide to go more yellow. get some small inexpensive sample bottles at the paint store, paint some poster board and hang it on a wall for couple of days because getting the right buttery yellow can be hard. A soft yellow and lavender is pretty together but I also like the sea green. When you said a combo of modern and cottage - that made me think of beach cottages, which often have soft colors (including sea green) and unfussy, calm interiors. You can put a large painting up if you have the wall space for it. Many bedrooms have tall chest, dresser mirror, tall headboard and big TV, all which take up wall space. If you can hang the painting without it being crowded by any of the things I mentioned, it should look very nice. If you wish to do a curtain fabric treatment to a wall and you have the vision of what you want - then go for it. I had a friend who did that in collage - only problem was the yards and yards of fabric needed got rather expensive. So, I would start with finding bedding you love. Simple white is good, and not too expensive. However, you might find something else light that you love, or a quilt or throw for the bottom of the bed and the colors and shades might lead you in a different direction for wall color or accents. So bedding first, then curtains, then see how your wall color is and then add accents and art - although your painting can also be an inspiration for colors.

You could copy this idea -- in your own lovely rich colours, and with your own scatter motif. Lots of painted dressers on this website: http://mohodir.blogspot.com/2015/04/painted-dresser-ideas.html

No experience with babies actually - but seems there are endless combinations - this was just the first / type in and select "Stories" for good articles like this with at least 2 ideas. Many "Photos" on houzz. "Shop" too. [https://www.houzz.com/magazine/guest-picks-desert-beauty-for-the-babys-room-stsetivw-vs~29205805[(https://www.houzz.com/magazine/guest-picks-desert-beauty-for-the-babys-room-stsetivw-vs~29205805)

The dark blue and white combination at no.1 looks like a perfect combination of colors, furniture, furnishings and accessories to give it a complete and cozy look.