A great question I enjoy bringing up is why are things, in general, "darker when wet". This applies to porous and granular materials like wetted stone, paper, sand etc. It also comes up in painting where "oiling out", applying a thin layer of medium over a dry painting, darkens most colors and makes them more saturated. There are many good explanations of "darker when wet" as, essentially, refraction index matching so that light has a longer stochastic path length though the absorptive medium (a "deeper" U turn).
Some paints are actually darker when dry, the effect doesn't seem to be as strong but it is certainly true. Acrylics generally dry darker: https://www.artistsnetwork.com/art-mediums/acrylic/acrylic-painting/
Some watercolors: https://www.handprint.com/HP/WCL/cds.html
And this seems to happen in oils too. I am in the middle of a discussion right now on how this, according to some accounts, happens more in mixtures of titanium white. Note pure titanium white seems to be the same value wet or dry; values of mixtures seem to shift more with it.
I can explain acrylics in a handwavy way, since the polymer emulsion probably scatters more than the film => drying reduces scattering and makes colors darker. I can't explain watercolor shifts or oil. In those cases when the medium (water or oil) leaves the paintlayer, the light should hit the particles more directly and there should be a greater difference in refractive index => more scattering => lighter color.
Apart from just jazz hands of non-idealities, does anyone have a good explanation as to how paint might dry darker? It would seem that dry and darker paint would somehow be making the light spend more time in the absorptive medium...I can't explain it in a way that would be consistent with why things are darker when wet. Is there another factor that isn't being accounted for in terms of refractive index changes across mediums?
Update: I'm virtually certain the answer is a well known paint film defect known as "flooding". It is seen as uniform darkening due to vertical separation of a (binary) pigment mixture. It is dependent on various factors including "wetability" of pigment particle surface, specific gravity but most importantly pigment particle size. Titanium white and carbon black (which are very fine, even though it looks like some sources report agglomerate sizes). I presume convective transport (volatile components in the case of oil paint) drive the separation. More here: https://books.google.com/books?id=T6XzCAAAQBAJ&pg=PA77&lpg=PA77&dq=paintfilm%2Btitanium%2Bwhite%2Bfloating&source=bl&ots=rvPQyHeJE5&sig=ACfU3U15E2jDFGgdfh4YxJwD6-SgK_9G9g&hl=en&sa=X&ved=2ahUKEwjzn9OoxMTvAhVS-qwKHQHTDh04ChDoATACegQIExAD#v=onepage&q&f=false