@ missile
I did some research this morning and tweaked the scene's lighting intensities to be more physically correct. In addition I also baked the light which fixed a few problems I noticed with previous dynamic-only setup. While the scene no longer matches yours the values are more correct, the reflections are a little more accurate (UE's reflection probes only capture light when using lightmass) and most noticeably the extra light has slightly shifted the colors slightly.
The images use the same fixed exposure settings. I swapped the black fog for an atmospheric fog as the black fog was causing weirdness with the bakes.
...
Looks better. But there is an old saying; a reflection is only as good as the
environment it reflects. So, yeah, it looks better. But I'm still not
convinced about the gold reflection. There seems to be some inconsistencies
and the question is where in Unreal (assuming my one is correct xD) the
problem occurs. I still think it's some form of post-processing which offsets
the color of the reflection.
I think we should render are more easy setup without any post-processing to
see whether we are able to reproduce the specular color at all.
Here is my new setup;
backround color: (1.0, 1.0, 1.0)
ground plane: size 2x2, color: (1.0,1.0,1.0) and (0.5,0.5,0.5), diffuse 0.5
sphere: center (0.0, 0.25, 0.0), radius 0.25, diffuse 0.0, specular color
gold (1.0,0.766,0.336), roughness small (make the spot small at least)
light: position (0.0, 2.0, -1.0), attenuation off
camera: position (1.0, 0.8, 1.0), lookat (center of sphere)
HDR: off
gamma: off
The only thing left on, on my end, is the the RGB clipper clipping off RGB
values (hard) to 1.0, which is the case for the highlight in my example, but
isn't important for our case unless the highlight has a long "tail" covering
the sphere to a greater extend changing colors. Hence, keeping it small won't
have any impact for the measurement to come.
Given this setup the center pixel has a viewing angle of 0, i.e. dot(V,N)=1
(make the triangles small enough). Under these conditions any Fresnel
implementation should reproduce the specular color, i.e. the F0 reflectance of
the material, in our case: (1.0,0.766,0.336), or RGB(255,195,86) (truncated).
The color of the center pixel in my rendering is RGB(255,195,86), verifying
the computation.
Whatever the lighting, post-processing etc., if this specular color can't be
reproduced at normal incidence, the colors of the reflection will be off no
matter how cool the environment. Hence, if you can reproduce this specular
color, anything else is just some fancy addon to direct the representation of
the scene.