I shaved with particles, and in visualization mode, so much eeve, so many cycles, it looks normal.
When I render, the problem appears, in both configurations. The beard becomes thick and dark. Does anyone know what can it be?
First off, welcome to the community, @Boo_Oliveira! My money is on the children setting in your particle system has different values under the “Display Amount” and the “Render Amount”. The Display Amount is how many show in your 3D viewport and the Render Amount is the number of those rendered. If you want them to look the same, make the values in those fields match. But that is really only if you want to use children particles.
Take a look at the manual page to read up on child particle options:
https://docs.blender.org/manual/en/latest/physics/particles/emitter/children.html#common-options