I've read so many articles on weeping willow bonsai & leaves and branches turning yellow. And I've come to the conclusion that I'm over fertilizing. I feed her pond water, then once a week 20-20-20 fertilizer. Too much? I'd say it's 10% yellow. Or am I worrying about natural cycle of new growth?

Looks OK to me. Maybe a little yellow. Do you feed it enough nitrogen? These plants needs fertiliser and water. Lots of it.

Has it been cold and ainy there for a bit? The plant can also respond to that. It is hard to tell from a picture, not knowing how it has changed. But this seems fine to me.

Leatherback, thank you. I do give her lots of pond water twice daily. But this morning I noticed all the yellow leaves had blown off, there's new buds of growth. It's my first attempt at a willow bonsai, I hope.

I saw a youtube video about willow bonsai with a wizz in bonsai. Him (like you) tried a lot of things and the best alternative was to put under the pot a plastic container full of water. Willows grow near lakes or sea and often have the roots in water so no risk of root rotting with willows.
I want to do a willow myself but I will wait until spring to collect.
Good luck.