Colors

Could you elaborate on that a bit? Maybe add some screenshots as well...

...a lot also depends on the lighting.

Here is the same Cube, with the same Material, in different lighting:

Yeah sure.

What I told you.

There is a difference between the Base Color of a Material and the way it looks when lit.

The (Base) Color is what colors of the light are being reflected of the Surface. Here's a fun one, a pure blue Cube, lit only by a pure red light:

The other parts of the (Principled or other) Shader tell how light is being reflected.

The behavior of light is complicated...luckily, you don't have to learn all about it to make beautiful renders.

If you want to know more about what happens, I can highly recommend the Fundamentals of Lighting here on CGCookie:

https://cgcookie.com/courses/fundamentals-of-digital-lighting-in-blender

You can always go for a more neutral light like this one if the color shift is too distracting for you.