I have some favorites for dogwood understory, but I'm afraid they wouldn't survive the winter in Ontario. A few details might help though: how much sun do you have in the target area? I imagine the cedars would provide a fair amount of shade once established, but how much does the area get now? My other question is how much protection will you be able to give them in the winter? I would think most "typical" hydrangeas (serrata and macrophylla types) would need *a lot* of protection to make it through a Canadian winter. But to get started, I would look at Hydrangea arborescens ('Annabelle' type) and Hydrangea paniculata ('peegee'), as those would be the best bets for hardiness. They are unfortunately more limited in terms of color and structure, but there are certainly some beautiful choices there. I'm sure there are others who will have great ideas, but I hope this at least gives you somewhere to start!

complEment for this usage- to make something look nice.
complIment if someone says something nice!!
btw, i made this very same mistake recently so i'm laughing to myself here, not at you.

sorry, my mother(spelling police) made me do it!

Try Hydrangea paniculata "Vanilla Strawberry." It should look good with any Dogwood, pink or white.