Is it permissible for an employee to take his right of overtime pay without his manager's knowledge, if the manager does not grant him this right like he does to his other colleagues?

If you have a right and the manager prevents you from it without justification, then it is permissible for you to speak with the manager and demand your right. If he refuses, then go to higher officials. If they do not give you justice, then submit a complaint to the competent authorities. If all means fail and you find no way to obtain your right, then it is permissible for you at that point to take your right without the manager's knowledge, provided that the right is firmly established for you, and that you have been unable to obtain it through all means, and that you do not take more than your right.