Q.Gingko biloba tree
Small, newly planted gingko tree – leaves are pale green to yellowing and beginning to drop. It is awful red clay soil so I mixed top soil with the clay & used starter fertilizer at the time the tree was planted (about 6 wks ago). I think it is too early for the fall color change but sure don’t know whats going on. I planted a 2nd gingko about 10′ away & it seems to be doing fine. We’ve had regular rainfall & the summer hasn’t been too hot so far. I have watered both trees equally when I thought it was needed. I need all the advice/help you can give as this tree is much to expensive to let die! Thanks.

Well draining soil is key to success with this tree.
Amendments for clay soil should include compost.
These articles will help you.
https://www.gardeningknowhow.com/ornamental/trees/ginkgo/ginkgo-tree-care.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/how-to-improve-clay-soil.htm