There are a few reasons why this could be happening. Listed as follows are some, with their solutions.
You are using EEVEE renderer instead of Cycles.
You don't have a material set.
You need to adjust the material set for your object to something other than the default.
I believe you are doing this already, but it is always good to *Compare your object material to another object in the scene, with Rendered view selected.
In these photos I compare the cube object with the helmet material to a blue Suzanne "monkey" head. (Also with a principled shader.)
One last tip: Adjusting world shader can sometimes give you a better preview of your materials. Here are examples of world brightness and color with a scene of objects with the helmet shader: Sometimes a dark world background with a bright light helps too:
I hope this helps!