private SolidColorBrush _bgcolor;
    public SolidColorBrush BGColor
    {
        get
        {
            return _bgcolor;
        }
        set
        {
            _bgcolor = value;
            RaisePropertyChanged(() => BGColor);
        }
    }

shell:SystemTray.BackgroundColor="{Binding BGColor}"

DataContext="{Binding Main, Source={StaticResource Locator}}"