Depends on the contract but the most I seen stipulate not working hour but context of the work. If it was made by instructions of the company for the company and you got remunerated for that then it is not yours to do whatever you please with it. Better check your contract in details.

Example: "All Intellectual Property Rights with regard to Developed Materials will be exclusively vested in and owned by the Company." (with additional data protection and confidentiality clause protecting company property)