Установите цвет вертикальной полосы в пункте меню Flyour

Вы можете установить его цвет в Platforms/Windows/App.xaml:

<maui:MauiWinUIApplication
    x:Class = "ShellFlyoutSample.WinUI.App"
    xmlns = "http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x = "http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:maui = "using:Microsoft.Maui"
    xmlns:local = "using:MauiAppTst.WinUI">

    <maui:MauiWinUIApplication.Resources>
        <StaticResource x:Key = "NavigationViewSelectionIndicatorForeground" ResourceKey = "SystemControlForegroundAccentBrush" />
        <SolidColorBrush x:Key = "SystemControlForegroundAccentBrush"  Color = "Red" />
        // you can set the other color such as blue/transparent
    </maui:MauiWinUIApplication.Resources>
</maui:MauiWinUIApplication>