For foliage colour as well as late summer flowers check out acteas. Womderful architectural shape and red/brown leaves. Would love heavy soil. Aconitums too would like it there. Astilbes and astrantias like heavy soil.
I currently have geum Totally Tangerine in flower. It will flower for months in a delightful orange colour. Get aster frikartii monch alongside and you will have a lomg flowering orange and blue association.
There are some fantastic orange echinaceas now but you will need to improve drainage locally for them a little.
Coreopsis too....yellows, reds and orange yellow.
For your orange/yellow scheme consider spireas like goldflame. Orange new shoots on yellow foliage. Pink flowers but I cut back a little to encourage brilliant foliage. Yellow and even orange foliage on Heucheras all year round.
Yellow and orange bulbs like daffodils,tulips and lilies will add splashes of colour