There are lots of plants that would work depending on your budget & time commitment to care for them. A simple way is to use a base of groundcovers that trail. Then add a few colorful accents as desired whether annuals or seasonal perennials tucked in then planted out in the regular ground garden. If you combine plants that tolerate the same level of moisture & growing conditions it's easy. I've used these evergreen or semi-evergreen plants in part shade containers without much work at all after planting except for some slug control, cutting back after flowering, and watering. Now & then I tuck the trailers back into the planters to fill out more or just snip off & plant elsewhere. Around Christmas I tuck in conifer boughs without damaging the plants. Creeping groundcovers: vinca minor, purple-leaved ajuga, golden creeping jenny, wintercreeper euonymus (slow growing Emerald Gaiety & Emerald & Gold), and Lamiums (not the invasive yellow bloomers, but the other ones). Japanese pachysandra has a sweet fragrant spring flower, so in a container you can smell it without bending over. Not showy after that, but green all year. Palace Purple Heuchera gives great contrast & colors up fine for me in morning sun. Then the airy stalk of white blooms give it a totally different look in summer. Bergenia Winterglut has great bronze fall & winter color with spring flowers. If you need more height and also want to please the hummingbirds the very hardy upright fuchsias from the Northwest Fuchsia society's list on the website will establish & return year after year. Some even have golden or variegated foliage to give more contrast. I mulch them every year in outdoor containers to be sure they return. I've added hybrid columbines from seed or serbian bellflower for more colorful blooms. Sometimes, even fringecup our native Tellima grandiflora. When the height of the bloom stalks in late May/June tend to flop a bit, but you can cut them off if desired or leave until fall. I've used our native lady fern with a variegated groundcover Bishop's weed, but that all goes dormant in fall then I cover with windblown conifer branches until the Bishop's weed regrows in spring. I haven't used other native plants in planters, but enjoy them in the gardens. Corrine

Powermuffin, thanks for the picture. I'm here on a search for pictures of cafes hung inside the frames, preferably 3/4 on double-hungs--but I didn't expect to get that lucky. Your windows look wonderful, and I think you've made up my mind to hang my summer curtains above the center to block more sun while still showing off the sky. We open our windows as much as possible, so I'll push the lower windows up to where their tops want to stop easily and hang the rods there. Nursekathleen, when I read your questions, Powermuffin's look was what jumped to my mind for setting off your wonderful old wood frames and simple and old-fashioned to suit your cottage. Of course, you probably noticed I really like that look. :)

I know this is an older post. Curious to know what colour your walls are? If pale, If white, I would paint the entire window black. If dark walls are what you have, I cannot see the colour...be careful as an all black window may be too dark for the space and a contrasting colour for the window casing may be a better fit. Post pictures if you are still trying to resolve this issue please.

Originally I thought Roman shades, but when I read "lightweight, whimsical sheers", I thought of Urban Outfitters, https://www.urbanoutfitters.com/shop/draped-shade-curtain?category=window-curtains-panels&color=004 and because all of their curtains are on the more whimsical side, I think you could pick any to use with a tension rod, because you don't have room for proper curtain rods, https://www.urbanoutfitters.com/window-curtains-panels