Skinceuticals c 10 serum skin staining issue.

how many minutes should you wait before applying a hydrating serum over the c 10 so doesnt effect ph level and absorption? now my issue, I use c 10, even on clean dry skin and apply hydrating serum 5-10 minutes after. the serum is pale yellow , but hours later my skin darkens to dark yellow/orange in spots where serum was applied. what is causing this? how can I make this not happen?