The property grid uses the ViewBackColor property to display the drop down back color. I don't see any other property that allows to change the drop down back color.

However, the control shown in the drop down is parented to a control (a Form) that you can modify, with a code like this:

public partial class MaskEditorControl : UserControl, IIntegerMaskControl
{
    private Color _initialBackColor;

    public MaskEditorControl()
    {
        InitializeComponent();
        _initialBackColor = BackColor;
    }

    protected override void OnParentChanged(EventArgs e)
    {
        base.OnParentChanged(e);
        if (Parent != null)
        {
            Parent.BackColor = _initialBackColor;
        }
    }
}