static void rebuildGrayColorTable()
{
    // 256 shades of gray.
    gray256.init(256);
    for (int num = 0; num < 256; num++) {
        int c = int(Palette::color[num].r+Palette::color[num].g+Palette::color[num].b)/3; //brightness

        int nearestColor = Palette::findNearestColor(
                        int(c * grayPercent),
                        int(c * grayPercent),
                        int(c * grayPercent));
        gray256.setColor(num, nearestColor);
    }
    gray256.setColor(255, 0);
}