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

Tolmiea menziesii--native piggyback plant would work, as it would trail over the edge.

Can you elaborate on your desire to limit these plants (preferrably) to NW natives? While some natives are highly ornamental garden additions, the vast majority are better suited to very natural, even woodland-type plantings. They tend to be big and boisterous and with a rather lmited bloom feature. Maybe not the best choices for a window box.

If you can depart from that requirement, you really are not very limited in your choices. There are some smaller evergreen type plants that can form the anchor or basis for your planter and all manner of seasonal plants to add color now or extend color into the colder months. Consider adding ferns or some shade grasses (Carex, Liriope) for textural contrast and whatever cascading groundcover of your choice.

Smaller decidious shrubs are a possibility as well and I know of several shady window box gardeners that add cut stems of colored twig dogwoods or curly willows for winter interest and various cut evergreens or holly during the holidays.