I've seen this happen in other areas of the editor as well: Data Table fields (floats) for example.
Manually typing in numbers to fields seems less prone to this error; using spinners to drag values reproduces the error more reliably
Sometimes a restart of the editor fixes it.
Using attached project:
-Open project in Editor
-Open .../Content/UI/LoadingScreen_Widget
-Select "Loading" Image Widget
-From Details Panel ->Appearance ->Color and Opacity, click the swatch to open the Color Picker.
-Use Eyedropper to click light blue color of Border Widget
-Attempt to PIE
RESULT:
Warning is thrown: "Can't Play In Editor when performing Edit Color and Opacity operation"
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There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-33580 in the post.