What about a butterfly bush. They come in shades of purple or a white that would compliment the color.

Hysop gets pretty large and it is purple -probably a good contrast. Hummingbirds love it. Devil's Dart is beautiful but won't get taller than the poker lily. Devil's Dart is a neon puple blue and gets about 3 1/2' tall.

have you thought about fennel or bronze fennel? both get tall, yellow flowers and catapillars feed on them.

The butterfly bush is a very good idea and so is the fennel be careful though in warm zones both can self sow to nuisance point so clip the flowers before they go to seed....... Eremurus foxtail lily in red or orange or yellow or white? if you want to capitalize on the kniphofia shape or yucca....... Clerodendrum speciosissimum glory bower is a red flowered shorter shrub that would look nice behind it...... I think it attracts butterflies it might be marginally hardy in your zone ....... Some of the salvias would be nice...... Salvia coccinea or mexican bush salvia or sage....... I think all salvias attract butterflies....... Euryops pectinatus is a nice plant that would work well....... Hibiscus rosa-sinensis is also a nice plant you could use yellow or white? or red....... it attracts hummingbirds...... but not butterflies......... something more tropical definitely..... you have alot of choices for you zone!!!!!! ........ :) ......