// Un-premultiply alpha before applying the color matrix. See issue #3539.
    if (color.a > 0.0) {
        color.rgb /= color.a;
    }

    color.r += diff;
    color.g += diff;
    color.b += diff;

    // Premultiply alpha again.
    color.rgb *= color.a;

    gl_FragColor = color;
}