Something like this:

    public static bool HasColor(this Bitmap img, Color color)
    {
        for (int x = 0; x < img.Width; x++)
        {
            for (int y = 0; y < img.Height; y++)
            {
                if (img.GetPixel(x, y) == color)
                    return true;
            }
        }
        return false;
    }