选择器的windows natvie控件是combobox。因此，您可以使用处理程序来禁用边框：

public App()
      {
            InitializeComponent();

            MainPage = new AppShell(); 
Microsoft.Maui.Handlers.PickerHandler.Mapper.AppendToMapping(nameof(Picker), (handler, view) =>
        {
#if WINDOWS
            var combobox = handler.PlatformView;
            combobox.BorderBrush = null;
            combobox.BorderThickness = new Microsoft.UI.Xaml.Thickness(0);     
#endif
        });
    }

或者你也可以通过改变windows自带的combobox样式来改变它的颜色。只需将以下代码添加到\Platforms\Windows\App.xaml：

<maui:MauiWinUIApplication
    x:Class="MauiAppTest.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:MauiAppTest.WinUI">
    <maui:MauiWinUIApplication.Resources>
        <ResourceDictionary>
            <SolidColorBrush x:Key="ComboBoxBorderBrush" Color="#Ff0000" />
            <SolidColorBrush x:Key="ComboBoxBorderBrushPointerOver" Color="#Ff0000" />
            <SolidColorBrush x:Key="ComboBoxBorderBrushPressed" Color="#Ff0000" />
        </ResourceDictionary>
    </maui:MauiWinUIApplication.Resources>
</maui:MauiWinUIApplication>

但是我不能重现你说的文字颜色改变的问题。您可以检查组合框的默认样式。