Make sure you balance your dark colors with plain green or they will receed so much in the shade (because I love those colors too, and well, it happens!!!). Hellebore is great for this kind of space (and you can get a white one).
I don't know if this amount of sun is going to be enough to keep golden foliage golden for you...and you can't force sun plants to grow and flower in a space where there is simply not enough sun. For a punch of color, you can always add impatiens, and try some nasturtiums in the area. They will at least fill in when other plants are small.
Ajuga with dark leaves would be nice, and how about some sweet woodruff to mix in. White flowers & dark green whorled foliage, bergenia, campanula Poscharskyana is one of my favorites, of course there are heucheras and hostas which can supply your foliage colors for you, lamium and creeping Jenny would be great along the ground floor, primulas and potentilla, violas would be pretty too...

for a very low front of the border planting have you thought of lanium or ajuga - both would give you a flower in the spring followed by pretty foliage the rest of the year .....

Lynne

hostas, bugloss (blue), astilbe (whites), trollius (yellow)

If you want something with a little more size and presence, I have a burgundy Ligularia that I love. It sends up spikes of yellow flowers in mid summer, but I have it for the foliage color. Primrose japonica is another shade favorite. The bloom time is early and to short, but stunning, the foliage is low and "cabbage-y"

Sounds like I'm just north of you - I love my dorconium right now (in a slightly sunnier spot). Daylilies (with reduced flowering), Astrantia, Bleeding heart (I know no pink, but I have a gold heart that flowers for only a few weeks and retains its yellow foliage), hydrangea, brunnera, black barlow columbine, toadlily, lady's mantle, goats beard.

How about Actea 'hillside black beauty' or Ligularia.

Maybe a clematis recta purpurea (non-vining clematis), calla 'flame', coleus, even Canna.

How much room do you have? How close is the tree? Will the plants be competing with tree roots?

Some of my fail proof shade/partial shade garden flowers: Wild (or Turkey) bleeding hearts (bloom spring - fall), Sedum, Hostas, Bottle Brush, Elephant Ears, Jack in the pulpits, Astilibe,Spirea, all ferns (for foliage). Have fun!

Thank you for all the suggestions!!

GGG - yes, I do have green foliage in there quite a bit as it is the most common, and I'm trying to throw in some lighter coloured flowers too (white, yellow) to balance it out. I learned the hard way that a bunch of darker colours all together just sort of blend together, lol!

There is a small spot along the pathway that gets more shade than the rest of the garden, and this is the spot that I'm trying to fill. The rest of the garden gets enough sun for most plants. But there are just a couple spots that get more shade. Won't be competing with the tree (it is far enough away) ...

Actaea is a good idea actually, I have "Hillside Black Beauty" which I didn't realize was a shade plant :) Perhapy it will be happier in a shadier spot. I also have a Ligularia that I bought last fall on sale with dark foliage, "Britt marie Crawford" I think.

A daylily perhaps, although I'm sure the bloom would be greatly reduced.

Ajuga looks quite nice, although many sources call it a groundcover, and that always worries me. I don't want something that spreads too much ... is Ajuga a fast spreader?

The thing I'm most worried about is putting Penstemon Husker Red or Actaea or Ligularia or another dark-foliage plant in this spot because it sounds like the dark colour fades more in shade to a green, and is most pronounced in sun. Does anyone have any experience with this? I don't want to take a beautiful foliage plant and then lose half the colour by giving it too much shade :)

Canna ... hmmm, that's an interesting idea too. Can Cannas take shade, I thought they were sunnier bulbs/plants as well?

What do you count as mid hight and what is too tall? Is the soil moist or dry? I like the idea of a ligularia too, the bugbane might be a bit too tall though...depending on what you are looking for.

Don't forget about the dark leaf perennial Geraniums (Midnight Reiter, Elizabeth Anne, etc. there was a thread on the Perennial forum not to long ago about the dark leaf kinds...)

PS. I have grown Penstemon 'Husker Red' in a somewhat shaded location (shade until about mid day) and the foliage color was more green with a bronze/purple overcast. Moved it to full sun last year and the foliage is much daker and vibrant!
CMK

I have Huskers Red penstemon in an area that gets about 4-6 hrs of morning sun and the leaves stay maroon/red. But the flower stalks get tall (about 4' tall).

I was also going to recommend geraniums like CMK suggested.

Candytuft (iberis) or phlox diverticata (sp?) might be another alternative.
I think Hostas might be a nice choice. there are so many cool colors.

A suggestion - Filipendula ulmarea aureum. Won't bloom much but the foliage a bright yellow green

link

http://www.mobot.org/gardeninghelp/plantfinder/plant.asp?code=B699

My torenia is in full shade and is doing very well. In fact I bought and planted several more today. You can get them in several colors.
kay

What about variegated oat grass, stays in a nice clump, or alchemilla, astrantia, golden alexandra lysmachia or pulmonaria, all fairly short and compact.
Flora

Just noticed the question about the penstemon.

I have my Husker Red in part shade (it gets about 2 hours of direct light and about 2 hours of filtered, dappled, light). It seems to be doing really well (this is its second year and one of them has doubled in size and the other one has put some good growth on too. It's quite a red colour but there is some green closer to the base - if I can, I'll grab you a picture later today. My ligularias don't fade to another colour either and they're in much deeper shade