Henryi is the first LFH to flower in my garden. Niobe usually opens 10 days later. Ville de Lyon is group 3 clemais and begins 3-4 weeks after Henryi. I'd suggest Asao, pink or Mrs.Cholmondeley, light blue, if you want something to flower at the same time with Henryi.

Pruning is easier if you don't plant a hard prune type 3 like V de L along with a light prune type 2 like Henryi because you can't tell which is which when they grow together.

nhbabs is right..planting like pruning groups will lessen frustration later on in years to come..you are fortunate that you are trying to chose something to compliment your clematis "Henryi"..being a white just any colour will compliment it..Your "Niobe" would be the one I'd plant with "Henryi" as they are both pruning group 2's and "Niobe" is a delicious shade of blackish deep red when opening fading to a deep reddish colour!!..
"Ville de Lyon" is one of my fav pruning group 3's..a fabulous and very vigorous white to plant with her would most definitely be clematis "Huldine"..I love those two so much that I have two of each...Jeanne