In previous builds, I think (or have I been dreaming?), after choosing Display Color Custom in the sidebar, the Colors panel would pop up automatically, if closed, as in the image below. (I generally keep it closed) In 516 it does not pop up when closed. Choose “Custom” and nothing happens. Such renders the sidebar useless for this operation. Personally, frequently I assign a custom colors or two to objects on specific layers to make them either stand out or fade contrasted with the primary layer color. That said, an even slicker method to quickly assign custom colors to objects on a layer would be welcome assuming there is a method available.
The work around is to use the Object Properties panel and click the color “rectangle” which does pop up the Colors panel.