名称空间中不存在该名称

我试图从C#代码中设置矩形的颜色。在其他主题中，帮助我使用DataContext，但不能绑定类颜色和密钥vmColors

主页.xaml:

<phone:PhoneApplicationPage
    x:Class="proba5.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:phone="clr-namespace:Microsoft.Phone.Controls;assembly=Microsoft.Phone"
    xmlns:shell="clr-namespace:Microsoft.Phone.Shell;assembly=Microsoft.Phone"
    xmlns:vm="clr-namespace:proba5" 
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d"
    FontFamily="{StaticResource PhoneFontFamilyNormal}"
    FontSize="{StaticResource PhoneFontSizeNormal}"
    Foreground="{StaticResource PhoneForegroundBrush}"
    SupportedOrientations="Portrait" Orientation="Portrait"
    shell:SystemTray.IsVisible="True">
    <phone:PhoneApplicationPage.Resources>
        <vm:colors x:Key="vmColors"/>
    </phone:PhoneApplicationPage.Resources>

    <Grid x:Name="LayoutRoot" Background="Transparent">

        <Grid DataContext="{StaticResource vmColors}">
            <Rectangle Fill="{Binding Gray}" />
        </Grid>
    </Grid>
</phone:PhoneApplicationPage>

colors.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace proba5
{
    public class colors
    {
        public string Gray
        {
            set { }
            get { return "#FF22262a"; }
        }
    }
}

为什么我名称"colors"在命名空间"clr-namespace:proba5"中不存在