In you Game class's constructor, add this line:
graphics.PreparingDeviceSettings += PreparingDeviceSettingsEventHandler;
And somewhere in your Game class, add this method:
void PreparingDeviceSettingsEventHandler(object sender, PreparingDeviceSettingsEventArgs e)
var pp = e.GraphicsDeviceInformation.PresentationParameters;
pp.BackBufferFormat = SurfaceFormat.Color;
The difference won't show up in the emulator. But on an actual device, it'll vastly increase your image quality by getting rid of a lot of your banding.