why are the leaves on my salix hakuro-nishiki turning brown after a few days

Hi Dariel and welcome - sorry lots of questions... When did you plant? Did you water? What soil is it in? Could well just be the leaf drop for autumn.

salix are generally known as very thirsty plants and that coupled with the onslaught of autumn will be the main reasons for the leaves.

If you've recently planted it, it'll be lack of water, I'd imagine. If that's a possibility, water it well - the leaves will drop fairly soon anyway, but you need to keep the plant alive!

I had this problem a few weeks ago. My salix is in a pot and I neglected to water it. After a good drink there are new green leaves appearing.
Happy gardening.