I have several dogwoods with different coloured stems. If you prune new ones down to about a foot, it gives a good framework to allow new branches to grow. You can then prune these the next year almost down to the framework as they will grow again. I have (and just have) pruned mine back in April even when new leaves are just showing and not had a problem. A s said, the current year's new growth is what provides the best coloured stems.