ContentDialog.xaml

<ContentView
  <ContentView.ControlTemplate>
    <ControlTemplate>
      <telerikPrimitives:RadBorder CornerRadius="2" BackgroundColor="{TemplateBinding BackgroundColor}">
        <Grid Margin="{TemplateBinding Padding}">
          <Grid.RowDefinitions>
            <RowDefinition Height="Auto" />
            <RowDefinition Height="*" />
            <RowDefinition Height="Auto" />
          </Grid.RowDefinitions>
          <Label Text="{TemplateBinding Title}" FontSize="Large" />
          <ContentPresenter Grid.Row="1" Margin="0,10" />
          <StackLayout Grid.Row="2" Orientation="Horizontal" HorizontalOptions="End">
            <telerikInput:RadButton Text="Cancel" Clicked="OnCancel"  />
            <telerikInput:RadButton Text="OK" Clicked="OnAccept" />
          </StackLayout>
        </Grid>
      </telerikPrimitives:RadBorder>
    </ControlTemplate>
  </ContentView.ControlTemplate>
</ContentView>

ContentDialog.xaml.cs

public partial class ContentDialog : ContentView
{
  public static readonly BindableProperty TitleProperty = BindableProperty.Create(nameof(Title),
      typeof(string),
      typeof(ContentDialog));

  public event Action<bool> Closed;
  
  public string Title
  {
    get => (string)GetValue(TitleProperty);
    set => SetValue(TitleProperty, value);
  }
  
  private void OnAccept(object sender, EventArgs e)
  {
    Closed?.Invoke(true);
  }
  
  private void OnCancel(object sender, EventArgs e)
  {
    Closed?.Invoke(false);
  }
}

MainPage.xaml

<Grid>
  <telerikPrimitives:RadPopup.Popup>
    <telerikPrimitives:RadPopup x:Name="popup" IsModal="True" Placement="Center" OutsideBackgroundColor="#6F000000">
      <local:ContentDialog Title="Please select a color" Padding="20" BackgroundColor="White" Closed="OnDialogClosed">
        <Grid>
          <Label Text="Red:" Grid.Row="0" Grid.Column="0" VerticalOptions="Center" />
          <Slider x:Name="redSlider" Grid.Row="0" Grid.Column="1" VerticalOptions="Center"
                  Minimum="0" Maximum="255" />

          <Label Text="Green:" Grid.Row="1" Grid.Column="0" VerticalOptions="Center" />
          <Slider x:Name="greenSlider" Grid.Row="1" Grid.Column="1" VerticalOptions="Center"
                  Minimum="0" Maximum="255" />

          <Label Text="Blue:" Grid.Row="2" Grid.Column="0" VerticalOptions="Center" />
          <Slider x:Name="blueSlider" Grid.Row="2" Grid.Column="1" VerticalOptions="Center"
                  Minimum="0" Maximum="255" />
        </Grid>
      </local:ContentDialog>
    </telerikPrimitives:RadPopup>
  </telerikPrimitives:RadPopup.Popup>

  <BoxView x:Name="box" WidthRequest="100" HeightRequest="100" HorizontalOptions="Center" VerticalOptions="Center" Color="Accent" />
  <Button Text="Select color" VerticalOptions="Center" Margin="20,140,20,0" Clicked="SelectColor" />
</Grid>

MainPage.xaml.cs

private void SelectColor(object sender, EventArgs e)
{
  SetColor(box.Color);
  popup.IsOpen = true;
}

private void OnDialogClosed(bool accepted)
{
  popup.IsOpen = false;
  if (accepted)
    box.Color = GetColor();
}

public Color GetColor()
{
  return Color.FromRgb(this.redSlider.Value / 255, this.greenSlider.Value / 255, this.blueSlider.Value / 255);
}

public void SetColor(Color color)
{
  this.redSlider.Value = color.R * 255;
  this.greenSlider.Value = color.G * 255;
  this.blueSlider.Value = color.B * 255;
}