As long as you are not using company resources, including company time, or producing something that competes with the company (effectively using inside knowledg

As long as you are not using company resources, including company time, or producing something that competes with the company (effectively using inside knowledge, another company resource), in most legal jurisdictions they can't stop you and any contract clause that tries is not legally enforceable. Of course that doesn't mean many don't try block such things…