The spring part of your question could I suggest primulas such as woodland dell, Ajuga reptans 'Catlins' Giant' Anemone lipsiensis and Cowslips? and for later what about Cyclamen hederifolium, and some of the Ferns- hearts tongue, lords and ladies, Trillium cuneatum -Wood Lily and finally Drimys lanceolata - Aromatica or Mountain pepper? When you say 'discordant note' its really down to your taste and the look you like.

I just have to jump in here and recommend one of our favourites, Arisaema (Cobra Lilies). They are summer flowering bulbs so are dormant when your spring flowers are out. Ours are in mid-season now. There are a number of species varying from a few centimeters tall to over a meter.
For more information and to buy you could do worse than visit www.jacquesamand.com
John Amand has a nursery in north London.

Just looked them up now I know what Jack in the Pulpit looks like! are they spreaders BH?

The bulbs multiply slowly, D, but they can self-seed quite readily. We have two really large clumps of A. ciliatum that have multiplied by dropping seed which has grown. The seeds are heavy so don't blow about all over the garden. Others multiply on their own as well and they are relatively easy to grow yourself from collected seed.

Thanks Bh

Arisaema are stunning plants and an absolutely essential part of any woodland garden, but be very careful where you source them from - large numbers are stripped from the wild in Asia and sold in the UK by "bulb merchants" (Arisaema grow from tubers rather than bulbs incidentally). Many species are extremely difficult to reproduce in cultivation where others come readily from seed, as BH says, as well as from offsets. So only source them from nurseries that are reproducing from their own stock rather than importing wild tubers.

Other great woodlanders that come into their own after the early Spring plants include Polygonatum & their close relatives Disporum & Maianthemum (formerly Smilacina) plus Kirengeshoma, Saruma, Anemonopsis, Tricyrtis, Corydalis, Roscoea (as in my avatar!) various woodland Lilium species & many fern genera - all wonderful and quite diverse plants.

I wholeheartedly agree with your comments about tubers stolen from the wild, Ilex. I have known John Amand for many years and to the best of my belief his stock is nursery grown.