How to consistently set EditText Selected Underline Color Programmatically

Modify your code in XfxEntryRendererDroid ControlOnFocusChange method like this :

private void ControlOnFocusChange(object sender, FocusChangeEventArgs args)
{
    _hasFocus = args.HasFocus;
    if (_hasFocus)
    {
        ...   

        EditText.PostDelayed(() =>
            {
                //Add the following code
                SetUnderlineColor(GetActivePlaceholderColor());
                EditText.RequestFocus();
                manager.ShowSoftInput(EditText, 0);
            },
            0);//Change it to 0
    }
    ...
}

Effect.

So the solution for me was to go pure AppCompat

So I'm adding an AppCompatEditText to the TextInputLayout

protected EditText EditText => Control.EditText;

protected override TextInputLayout CreateNativeControl()
{
    var textInputLayout = new TextInputLayout(Context);
    var editText = new AppCompatEditText(Context)
    {
        SupportBackgroundTintList = ColorStateList.ValueOf(GetPlaceholderColor())
    };
    textInputLayout.AddView(editText);
    return textInputLayout;
}

Then from there I was able to set the underline consistently with this.

private void ControlOnFocusChange(object sender, FocusChangeEventArgs args)
{
    _hasFocus = args.HasFocus;
    SetUnderlineColor(_hasFocus ?  GetActivePlaceholderColor(): GetPlaceholderColor());
} 

private void SetUnderlineColor(AColor color)
{
    var element = (ITintableBackgroundView)EditText;
    element.SupportBackgroundTintList = ColorStateList.ValueOf(color);
}

full source code here.

Why don't you change the tint colour at runtime using this (May be in your text changed event):

ViewCompat.SetBackgroundTintList(_YourView , ColorStateList.ValueOf(Color.ParseColor(Resources.GetString(Resource.Color.blueLine))));

Anyways Goodluck!