How can I allow player to pick color? (v4.21.2)

There no picker widget for UMG, either oyu doing something yourself, here some tutorial i found (remeber you making UMG widgets, so oyu can make one for picking color and reuse else where):

Or use if C++ is not issue for you, make UMG widget based of Slate widget SColorPicker

It in Runtime folder so you can use it in game