So how much direct sun does it get under the trees? If it's not too much, you might try Lamiastrum which has pretty variegated leaves. I have a bed of it that gets some sun and does well.

golden creeping jenny (moneywort) or bronze glow ajuga all do well with partial sun. I have some of each getting a good chunk of their sun in the afternoon and they are doing great. Both look decent all winter too.

I like Caitlin's Giant ajuga when it blooms in the spring. It's a little pachysandra looking the rest of the year. I like the bronze ajuga too, but it grows in unexpected ways for me. It will completely die out where I plant it and turn up a few feet away. It's like it jumps when my back is turned.

How about hellebores? I keep hearing that although they tolerate shade, they like sun. They certainly are gorgeous this time of year.

How about just plain mulch? Sometimes less is more pleasing.

I have some creeping Jenny that seems to love the adfternoon sun. It was stressed in full sun, but doesn't seem to mind the afternoon sun. In fact, It really spreading... fast!

There are also plenty of Hardy Sedums that can provide a great ground cover, although they are a bit tough to remove leaves from in winter. The crepping Jenny is also a problem removing leaves that fall. I use a vacuum to remove leaves from mine.