...
// Fake gravitational blueshift //redshift
void    SkyBlueShift(inout vec3 color) //SkyRedShift
{
    float onePlusZ = inversesqrt(1.0 - gravRadius / EyeDist);
    float onePlusZ2 = onePlusZ * onePlusZ;
    float flux = onePlusZ2 * onePlusZ2;
    float shift = (onePlusZ - 1.0) * 2.0;

    const float r2g = 0.299 / 0.587;
    const float g2b = 0.587 / 0.114;
    color = mix(color, vec3(0.3*color.r, color.r*r2g, color.g*g2b), clamp(shift,     0.0, 1.0));
    color = mix(color, vec3(0.7*color.r, color.r*r2g, color.g*g2b), clamp(shift-1.0, 0.0, 1.0));

    color *= flux;
}
...
#ifdef ENV_MAP
        // obtain color from the skybox and the frame texture
        vec4 colorSkyE = texture(EnvMap, SkyRot * rayDir);
        vec4 colorSkyF = texture(Frame, rayDirF);

        // if pixel is outside the frame edges, or camera is close to the event horizon, use the skybox
        float blendZone = 0.05 * clamp((0.2 - EyeDist) * 10.0, 0.0001, 1.0);
        vec2  d = (rayDirF - clamp(rayDirF, blendZone, 1.0 - blendZone)) / blendZone;
        float blend = clamp(1.0 - dot(d, d), 0.0, 1.0) * blendFactor;

        // mix skybox and frame color based on distance to make smooth transition
        vec4 colorSky = mix(colorSkyE, colorSkyF, blend);
#else
        // obtain color from the frame texture only, mirror-repeated to fill gaps
        vec4 colorSky = texture(Frame, rayDirF);
#endif

        // fake blueshift the sky image based on proximity to the event horizon //redshift
        SkyBlueShift(colorSky.rgb); //SkyRedShift

        // take into account absorption of the accretion disk
        color += transm * colorSky;
    }

    OutColor = color;
}

#endif