The password is yours. Consider it like your toothbrush: never share it and change it regularly. There is no legitimate reason for you ever to be asked to disclose your password -- not by the CERN Computer Security Team, the CERN Service Desk, or your colleague. In fact, sharing your password is a direct violation of the CERN Computing Rules, as set out in Operational Circular No. 5. Your colleague may not be aware of this, but you should bring it to his/her attention as you politely decline.