Get Average Colour From Texture?

To calculate the brightness of a color you could try this method and then calculate the average brightness of all returned values (not tested):

 private int Brightness(Color c)
 {
     return (int)Mathf.Sqrt(
        c.R * c.R * .241 + 
        c.G * c.G * .691 + 
        c.B * c.B * .068);
 }

@Edit

To get the pixels of a Texture2D just call GetPixels() or GetPixels32()

 Texture2D destTex = new Texture2D(width, height);
 Color[] pixels = destTex.GetPixels();

sources: http://www.nbdtech.com/Blog/archive/2008/04/27/Calculating-the-Perceived-Brightness-of-a-Color.aspx http://docs.unity3d.com/Documentation/ScriptReference/Texture2D.GetPixels.html