public static Color getDominantColor(Bitmap bmp)
        {
            //Used for tally
            int r = 0;
            int g = 0;
            int b = 0;

            int total = 0;

            for (int x = 0; x < bmp.Width; x++)
            {
                for (int y = 0; y < bmp.Height; y++)
                {
                    Color clr = bmp.GetPixel(x, y);

                    r += clr.R;
                    g += clr.G;
                    b += clr.B;

                    total++;
                }
            }

            //Calculate average
            r /= total;
            g /= total;
            b /= total;

            return Color.FromArgb(r, g, b);
        }

http://www.vcskicks.com/dominant-color.php

Extract Dominant Image Color

C#