Is it permissible for me to leave work early or reduce my evening work hours as compensation for working alone for longer hours in the morning due to my colleague's absence?

It is not permissible to breach the agreed-upon official working hours except with the permission of the authorized employer, based on the Almighty's saying: "O you who have believed, fulfill [all] contracts," and the Prophet's (PBUH) saying: "Muslims are bound by their conditions." Therefore, you must check the work regulations to determine whether you are entitled to reduce your working hours due to your colleague's absence.