Take it from anArtist--the complimentary color of yellow is mauve and the color next to yellow on the color wheel is orange who's complimentary color is blue. Since many blues are mauve based those 3 colors work well together. In fact I have a bed that contains those colors plus variations of those colors and it looks good. I tend to go for long blooming or reblooming perennials

Goblin Gillardia has large orange red daisy like flowers tipped in yellow. It grows 12 to 18 inches tall.

I have stella de oro day lilies and a peach one called mini pearl t6hat are both reblooming

I have correopsis moonbeam which has small lemon yellow daisy like blooms and feathery foliage.

I have tulips for spring planted in clumps in the bed and I surround them with annual purple petunias The petunias spread out and cover the space where the bulbs are that would otherwise be bare after the foliage goes.

I have long leaf veronica with it's spikes of purple flowers

I have annual forget-me-nots that reseed themselves.

Along the edge I have a variety of blue and white carpathian bellflowers called clips It is interspaced with a low growing purple aster. The clips bloom first followed by the asters which bloom into fall.

As you can see it is a rather big bed and you may not be able to use all of these but I hope you can find some you can use

Hi Kelsey,
My 'Summer Suns' are kind of spindly this year so I can't show you anything (we are in a wicked drought here so quite a few of the guys are affected) but I have a bed with rosy coneflowers (which the Japanese beetles chew up pretty good-grrr), pink Asiatic lilies, 'Walkers Low' nepeta-catmint which are still blooming a bit, 'Grand Marshall' monarda-bee balm which is a vibrant medium pinky purply color; in the back of everybody I have a 'Royal Purple' cotinus-smoke bush, 'Diablo' physocarpus-ninebark, a little blue spruce on the side, Karl Foerster calamagrostis and 'Sapphire' blue oat grass, and in front I have several heuchera-coral bells, a couple orangey ones and a couple purply ones, a 'Bonfire' euphorbia, and a gorgeous little rue, 'Jackman's Blue' and a gold Mops chamaecyparis, which is beautiful, vibrant gold year round...
I digress from your question I guess-I love blues too, that's why I have the blue oat grass, spruce, and rue; I don't know where you are, but most of the bluish perennials are done for me, plus I need to try to keep from the ones the J.beetles love...tho I do want to add some 'Blue Paradise' Phlox to this bed, which are bluish purple.
I am really bummed that my heliopsis is so lackluster this year. Sorry I babble so...

Thanks so much, so many ideas -- away to the garden center I go. . . .

I just found out these are what I have in my new yard. I was searching for more info and images of the heliopsis and found a picture of heliopsis with campanula sarastro. The purple/mauve color did look amazing with the bright yellow. So I reiterate what dowlinggram said. Yellow and purple love one another. Especially, in my opinion, in contrasting tones. Bright yellow, with a powdery, paler purple. Perfect.

I am a sucker for the purple (or blue to some)/yellow-orange combo as well, so platycodon grandiflorus (balloon flower) makes a nice combo. I also like the combo of bee balm with heliopsis. Caryopteris, Sea Holly (eryngium), Joe Pye Weed(eupatorium maculatum), and Jupiter's Beard (centranthus ruber) would all look nice with 'Summer Sun.'

There are dwarf cultivars of most of the companion plants mentioned above so you can easily find something that will work in front of 'Summer Sun.' Good luck!

I have summer sun with blue salvia on one side and orange lilies and red orange daylilies on the other side. In front is silver lambs ears which I think looks nice together as well.