We have 3 crabapples. Near the trunk and area we have Soloman Seal and False Soloman Seal ...then a bit further out but still within the drip line, Bunchberries and Wood Lilies.

we have blue ajunga under ours. it's a very hardy and spreading ground cover. we just mow around ours to keep it under the tree and a nice round shape.

I saw a nice planting at a new public building in a small town near me. They had used two crabs with leaf and bark coloring like you described. In the curve of the radius in front of one of the trees, they had underplanted with a mass of daylilies-most likely Stella D'Oro-blending into a group of small cotoneasters. To the other side, they had underplanted one wiegela with a mauvey-rose type bloom (maybe Wine & Roses), which picked up the leaf color of the crab very effectively. At the other end of the building behind the other tree, they backed it up with a row of three spirea shrubs-green leaves with a raspberry color flower. In between these two beds they used several Rose Glow Barberry shrubs, which also picked up the leaf color of the crabs very effectively. I know this was a public building, but this was a very pleasing affect-I was a little jealous I hadn't thought of any of these "companions" myself! Hope this helps.

I have pink species tulips under my two Robinson crabapples that bloom when the tree blooms - the trees are located in my fairly narrow parking strip so the warm cement may affect timing of bloom.
Even though the tree blooms briefly it is a fun combo if you like pink! The rest of the year kinnikinnick acts as a low maintenance groundcover, more plants would spice this up a bit when I get to that area!