调试易

        void GrayMenuClick()
        {
            Bitmap bmp = new Bitmap(picBox.Image);
            Bitmap newbmp = new Bitmap(picBox.Image);
            Color c = new Color();
            Color NewC;
            Byte r, g, b;
            int  gray;            for (int i = 0; i < bmp.Width; i++)
            {
                for (int j = 0; j < bmp.Height; j++)
                {
                    c = bmp.GetPixel(i, j);
                    r = c.R;
                    g = c.G;
                    b = c.B;
                    gray = (int)((r * 0.9 + g * 0 + b * 0));
                    NewC = Color.FromArgb(gray, gray, gray);
                    newbmp.SetPixel(i, j, NewC);
                }
            }            // 刷新显示
            picBox.Refresh();
            picBox.Image = newbmp;
这里是代码。现在的问题是，我的背景是黑色的，里面的数字是白色。怎么让他们反转过来呢。背景变成白色，数字变成黑色