Dislaimer: There's a lot of hits when googling my issue. I've spent over a day to review all I could find. Regrettably, those can be placed in two categories. First one corresponds to the interop based solution and the other to the VSTO based one. None of them applies in my case.
I've found this tutorial and this documentation. Still, when applying those (in different variations), the green rectangle in my cells keep appearing.
It's not my decision to use EPPlus but it's my responsibility to make the green thing to go away.
How can I do that and how should I google future issues with that library (it seems like I'm doing something wrong)...
I've tried stuff like this (amongst many others, of course).
var worksheet = package.Workbook.Worksheets.Add("Something");
var validation = worksheet.DataValidations.AddListValidation("a1:aa99");
validation.ShowErrorMessage = false;