Dynamically change gradientstop color
<ListBox ItemsSource="{Binding lbox}" Name="listboxUsers" Margin="2,126,0,6" Grid.Column="1" Grid.ColumnSpan="2" Grid.RowSpan="2">
<SNIP>
    <Rectangle Height="100" Width="100" Margin="12,0,9,0" Name="Recta">
        <Rectangle.Fill>
            <LinearGradientBrush EndPoint="1,0.5" StartPoint="0,0.5">
                <GradientStop Color="Gray" Offset="1"  />
            </LinearGradientBrush>
        </Rectangle.Fill>
      </Rectangle>

public class lbox
{

    public lbox(string par1, string par2, Color par4)
    {
        displayname = par1;
        state = par2;

        gscolor = new GradientStop();
        gscolor.Color = par4;
    }

    public string displayname { get; set; }
    public string state { get; set; }
    public SolidColorBrush lbcolor { get; set; }
    private GradientStop gscolor { get; set; }
}

GradientStop Color="{Binding gscolor}" Offset="1"  />

<Rectangle Height="100" Width="100" Margin="12,0,9,0" Fill="{Binding lbcolor}">