I like a contrasting color as neighbor: Like the chartreuse leaves of Ipomea Marguerite circling the feet of Redleaved Canna, or the golden frilly leaves of Tanacetum vulgare 'Isla Gold' with a dark leaved Coleus spilling underneath.

If you have a low purple-leaf shrub like Red Barberry, maybe a silver Lamb's Ears beneath with one of the tall silver-blue ornamental grasses for background.

I think some dark colored leaves look good with the late afternoon sun shining through/behind them...I have some dark red Coleus that just glows. I grow a lot now in containers so I can move things around, but I used to sit a new plant out in garden for few days before planting..just to be sure how it would look. Also, place your plants for best viewing at the time of day you'll be in garden most often. For me it's dawn or late afternoon. At dusk a tall white Artemesia looks like lace in front of Cotinus' dark purple leaves. Neither would be as effective apart.

You might not like variegated plants...I didn't until I saw how a bit of white striping on a houseplant leaf perked up all the plants around it. Now I look constantly for colored or variegated foliage, (but I've got 30 years' worth of green plants to perk up! :)
josh