My latest 'jolly' plant (my euphemism for one that might be a bit too aggressive) is Euphorbia Fens Ruby (or Ruby Fens, I always forget which way). It spreads quite happily, has good foliage spring through fall, flowers in mid-summer, and has good winter seedheads. I'm rather liking it and plan to let it colonize at the top of a slope I'm working with. No clue how it might interact with the existing pasture grass, but we shall see.
Another thought is arnica which may be native for you, and/or borage for a blue pop.
