Use gradients to set the value of the Background or Fill property of elements. You can change the color as well as transparency in the gradient definition.

A gradient is defined using a vector and a set of gradient stops. Each gradient stop has a position from 0 to 1 (0% to 100%):

Example: Define a simple LinearGradientBrush:

<!-- gradient defined directly in an element-->
<Rectangle Width="300" Height="200">
    <Rectangle.Fill>
        <LinearGradientBrush StartPoint="0.5,0" EndPoint="0.5,1">
            <GradientStop Color="Red" Offset="0" />
            <GradientStop Color="Yellow" Offset="0.4" />
            <GradientStop Color="Green" Offset="0.6" />
            <GradientStop Color="Blue" Offset="1" />
        </LinearGradientBrush>
    </Rectangle.Fill>
</Rectangle>

<!-- gradient defined in resources-->
<Page.Resources>
    <LinearGradientBrush x:Key="GradientBackground" StartPoint="0.5,0" EndPoint="0.5,1">
        <GradientStop Color="Red" Offset="0" />
        <GradientStop Color="Yellow" Offset="0.4" />
        <GradientStop Color="Green" Offset="0.6" />
        <GradientStop Color="Blue" Offset="1" />
    </LinearGradientBrush>
</Page.Resources>
 
<Grid>
    <Rectangle Width="300" Height="200" Fill="{StaticResource GradientBackground}" />
</Grid>

Example: Create gradient stops programmatically:

<Rectangle Loaded="Rectangle_Loaded">
    <Rectangle.Fill>
        <LinearGradientBrush StartPoint="0,0" EndPoint="1,0" />
    </Rectangle.Fill>
</Rectangle>

using Windows.UI;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Media;
using Windows.UI.Xaml.Shapes;
 
namespace TestApp
{
    public sealed partial class MainPage : Page
    {
        public MainPage()
        {
            this.InitializeComponent();
        }
 
        private void Rectangle_Loaded(object sender, RoutedEventArgs args)
        {
            var rect = sender as Rectangle;
            var brush = rect.Fill as LinearGradientBrush;
 
            brush.GradientStops.Clear();
 
            var s1 = new GradientStop();
            s1.Color = Colors.Blue;
            s1.Offset = 0.0;
            brush.GradientStops.Add(s1);
 
            var s2 = new GradientStop();
            s2.Color = Colors.Red;
            s2.Offset = 1.0;
            brush.GradientStops.Add(s2);
        }
    }
}