I'm using the default Diffuse BSDF for every material with custom color, but none of those colors are exported to the
.mtl file (they all appear to be the default color
rgb(0.8, 0.8, 0.8)).
However with the Principled BSDF shader I can export color values to the
.mtl file but everything is darker (like
rgb(0.9, 0.7, 0.5) becomes
rgb(0.7, 0.5, 0.3)).
(related question, but not solved: https://blenderartists.org/t/rgb-values-wrong-when-exporting-as-obj/1179228/6)