There is a beautiful purple "Million Bells" that is great in a pot -- gives lots of bloom and cascades over the side. You would have to get smallish marigolds in a rich gold colour to go with this deep purple. I think small variety because the big ones don't usually bloom until later in the summer if the weather stays cool. Pansies would be lovely, but you have to pick off the dead flowers or they will stop blooming. Million Bells don't need deadheading which is why I prefer them to petunias. The rule for pots is: thrillers, fillers and spillers -- spiky plants, fat filler plants and cascading ones. Sounds like a fun project. I'm sure others will have suggestions for 'thriller' plants.

You need potting soil in your pots. Not composted manure. Manure is a soil amendment, not a potting medium. Way too rich and will kill your plants.

Million bells and marigolds are excellent suggestions. If you have shade, then impatiens come in purple but not yellow. There are pansies that are purple and yellow, as well as purple, and yellow, and any other color. Mid June should be ok still for pansies.

How about the Tradescantia 'Sweet Kate' flowers Violet blue, foliage golden.

I think you should go with green and yellow. Just kidding.