There are loads of colour ways that you can jump in with that would be complementary. You just need to have a bit of a play and decide how much drama you would like at your window. You talk about cost as being a factor, and for the size of the window, I think you would find that blinds and curtains depending on your choice can come out at a similar sort of cost. Also as you have a lovely bay window, you may want to consider dressing the edges with 'decorative' curtains, that are styled, but never pulled. And then go for blinds for functionality. However - I would also consider the heating in your house. As a general rule, curtains retain more heat than blinds. If you want to discuss colours etc further, I'd be more than happy to have a chat. And please post what you go with in the end. I would love to see the finished article.

I like the see through effect of what you have at the moment, and I’m guessing you only need the curtain for privacy and softening, not blackout?. So I would suggest a heavy voile or unlined linen in an off white. Perhaps one single large curtain so it all stacks to the right where you have a larger wall stub. Modern heading (eg wave or euro pleat) and either a track or a very simple metal pole. I would steer clear of anything too heavy or dark.

The green curtains are actually darker than the walls so I don’t think you need an exact match. A nice deep forest green should do the trick. I’ve bought fabrics from Fashion Interiors in Coventry who also sell online if that's any help. I think they also make curtains. May be worth having a look and then order some swatches? www.fashioninteriors.co.uk

Your room looks lovely and the wallpaper is soft and pretty, perfect for a bedroom. If the company the paper is from do fabric in the same print that would look great for the windows and would make the scheme cohesive. It wouldn’t be too much as you have only papered one wall. I wouldn’t have short curtains though but roman blinds with thick interlining to make them look luxurious and provide insulation. I wouldn’t use navy at all, I think the green in the paper is a lovely soft colour and would be better for cushions etc. If you can’t get the matching fabric then use that green or the pale blue for the windows. As long as you stick to the shades in the paper it will all come together.