[RESOLVED] can't modify user control passed as parameter

In a VB.Net 2017 Windows Forms project, I've defined a user control (named "ExDatePicker") that includes a label, a textbox (named "txt") and a datetimepicker. In several places, I'm passing specific instances of that user control to the Sub shown below. This seems to work as intended for most of the instances, but for one of them it fails to change txt.BackColor. When I step through the code, it APPEARS to change the .BackColor value but that change is never reflected on the screen. At first, the user control parameter ("exdtp") was defined as ByVal and I thought that was the problem. But even after changing it to ByRef, the problem persists. Any ideas about what might be causing this?

Code:

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    Public Sub CheckDateTimePicker(ByRef exdtp As ExDatePicker, ByRef Message As String, Optional IsRequired As Boolean = False,
      Optional MinValue As Date = MinDateValue, Optional MaxValue As Date = MaxDateValue)
        Dim OldMessage As String, Temp As String, Dt As Date
        OldMessage = Message
        Temp = exdtp.txt.Text
        If Date.TryParse(Temp, Dt) Then
            If (Dt < MinValue) Or (Dt > MaxValue) Then
                AddText(Message, exdtp.DisplayName & " must be between " & FormatDate(MinValue.ToString) & " and " & FormatDate(MaxValue.ToString) & ".")
            End If
        Else
            If IsRequired Then
                AddText(Message, exdtp.DisplayName & " is required.")
            End If
        End If
        'set .BackColor to reflect whether any edit checks failed
        exdtp.txt.BackColor = If(Message = OldMessage, NormalBackColor, WarningBackColor)
    End Sub

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Nevermind, it turned out this Sub is working correctly, and txt.BackColor was being reset by some other subsequently executed code. And it even works correctly when exdtp is passed ByVal (I'm guessing because when you pass a reference type ByVal, its members may still be changed by the called code even though the reference itself may not be).