If the bed is viewed mainly from one direction and you can plant the iris in front of the tulips that might work at blocking the view of the yellowing foliage. I also like the idea of lady's mantle. If the tulips are in a sunny spot I don't think the lady's mantle will interfere with the tulip's sun. Right now I'm using sweet alyssum "carpet of snow" direct sowed right over my tulips. It's really cheap to just buy a pack of seeds if you like sweet alyssum. I don't have any experience with yarrow or coreopsis. Hardy geranium or catmint might look nice too.

Thanks for the reply, Kristin. The area is so small that whatever goes there really has to go on top, as opposed to in front. It is a small circle, about 3 feet across. Cat mint would be nice. Can your Sweet Alyssum do well on little water?

I pretty much ignore the sweet alyssum and it does fine. I just sprinkle the seeds right on top of the soil, I don't cover the seeds (I think they need light to germinate). I water some at the time of sowing, and until they start sprouting.

Here's a picture from a couple years ago. I think this was late summer/fall. The sweet alyssum is the white stuff.

I have been planting glads beteen the tulips when the wheather is warm enough and the tulips flowers are gone. By the time the glads start to come, the tulips will die back. Since you only need to plant glads 4 inches deep, you can dig them up carefully in the fall and save for the next year.