If you’re a Muslim, I’m happy to see that you’re doing this. Why? Because you are simply respecting the beliefs of another person. It’s much like respecting the beliefs of a fellow Muslim belonging to a different sect of Islam.
When it comes to religion, everyone has a certain set of beliefs and they have their own interpretation of those beliefs. It’s not necessary that everyone else subscribe to your set understanding or interpretation. Also, respecting someone’s beliefs doesn’t mean you are setting aside your own.
So, if you have a colleague who’s celebrating Christmas today, go ahead and wish them. You will earn their respect. Moreover, you will promote harmony.