Another excuse to use System.Linq

public Color GetAvgColor(Texture2D tex, Rectangle rect, int mipLevel = 0)
{
	Color[] data = new Color[rect.Width * rect.Height];
	if (data.Length == 0) return Color.White;
	tex.GetData<Color>(mipLevel, rect, data, 0, data.Length);
	int r = data.Sum(t => t.R) / data.Length;
	int g = data.Sum(t => t.G) / data.Length;
	int b = data.Sum(t => t.B) / data.Length;
	int a = data.Sum(t => t.A) / data.Length;
	return new Color(r, g, b, a);
}

edit with a bit of explanation: the 3rd overload of the GetData allows you to specify a rectangle to retrieve colours from, so you simply have to make sure your data array is the same size as what your after returning.

Another excuse to use System.Linq

public Color GetAvgColor(Texture2D tex, Rectangle rect, int mipLevel = 0)
{
    Color[] data = new Color[rect.Width * rect.Height];
    if (data.Length == 0) return Color.White;
    tex.GetData<Color>(mipLevel, rect, data, 0, data.Length);
    int r = data.Sum(t => t.R) / data.Length;
    int g = data.Sum(t => t.G) / data.Length;
    int b = data.Sum(t => t.B) / data.Length;
    int a = data.Sum(t => t.A) / data.Length;
    return new Color(r, g, b, a);
}