Hi, if I've got the right tree, you can find advice on how to prune the flamingo tree on this site:
http://midwestgardentips.com/hakuro-nishiki_dappled_willow.html

hope it helps

There are two plants referred to as Flamingo tree - one is Salix hakuro nishiki and the other is Acer negundo 'flamingo'. If your plant is the latter, then you can prune it now. If its the Salix, then it needs trimming back in spring, after the catkins have gone, or late April/early May, depending on how cold it is.

the link I gave above relates to Salix hakuro nishiki that Bamboo has mentioned.