Kerria, Kirengeshoma, Trycyrtis, a chartreusy hosta, a fine-leaved, creeping, chartreuse sedum with yellow flowers, all green Arisaema triphyllum, Rohdea, Tolmeia (these are all things kinda sorta near our 'Stoplight'. I'd say anything with strappy or spikey chartreuse foliage would be a good foil.

while the foilage itself may be green why not crocosmia 'Lucifer' with creeping jenny at the base....

Lynne

i think it would look nice next to amber waves heuchera. and/or maybe some of the purples- though you'd have to be careful. tam

Anything purple or red - I'd go for the taller Alternathera 'Dark knight or Black Knight or maybe its Black Night' one of those. It would even go well under Black Magic Elephant Ears or one of the dark Phormiums (but they like to dry out a bit more than a Heuch would). One of the dark red/purple Ligularias (I grew L. dentata but it finally bit the dust and never liked it here) or one of the Japanese Painted Ferns with a lot of purple or darkness in the foliage. I would let it be the chartreuse ellement and let the other plants play off the red or darken the stage so that the Heucherella really glows.

Brenda, I tuck spiderwort in between my heucheras and heucherellas. There's one variety that has pale lime green foliage and electric blue flowers. I like the effect and it blooms a long time in bright shade. The strap foliage is not as tall as regular spiderwort so I tuck it in between the other plants, rather than behind them.

The Oxalis with the wine-purple foliage might work with your plant.

oooh- what about purple heart? it's a wandering jew/tradescantia relative. some of the alternatheras have a deep wine foliage that would look nice, too. or black or mini mondograss. i was drooling over stoplight at pdn. such a gorgeous plant!