Fast luminance inversion (aka. color invert + hue rotate 180°)

luminv.c

// Fast luminance inversion.
int luminv(int rgb) {
    int r = rgb >> 16,
        g = rgb >> 8 & 0xff,
        b = rgb & 0xff;
    switch (((r > g) << 1 | (g > b)) << 1 | (b > r)) {
        case 4: // r > b > g
        case 3: // g > b > r
            return rgb + (255 - r - g) * 0x10101;
        case 2: // g > r > b
        case 5: // b > r > g
            return rgb + (255 - g - b) * 0x10101;
        default:
            return rgb + (255 - r - b) * 0x10101;
    }
}

luminv.py

def luminv(r,g,b):
    """Fast luminance inversion."""
    c = 255 - max(r,g,b) - min(r,g,b)
    return r+c, g+c, b+c