Is what the questioner did—lying to his colleague by saying he was unable to drink tea while fasting—considered unlawful and does it affect the validity of his fast, and is concealing his fasting from his colleagues permissible?

Your statement to your colleague that you are unable to drink tea is truthful, because an obligatory fast cannot be broken, due to the Almighty's saying: "And do not invalidate your deeds." Moreover, continuous fasting is a condition for the atonement fast for killing, due to the Almighty's saying: "so [he must fast] for two consecutive months." As for concealing your fast from them, there is no harm in it, especially if it is out of fear of their chatter and questions.