Am I sinful if I keep silent about a colleague of mine who takes money through illicit means?

You should advise your colleague kindly and explain the حرمة (prohibition) of his action, and that unlawful money has little blessing, great sin, prevents provision, and blocks supplication. If he does not respond, then report him to whoever has the authority to stop him from injustice, as long as you do not fear greater harms. If you do fear that, then continue to advise him and show your disapproval of his action. If shunning him would deter him, then shun him in a way that achieves theired interest and does not cause another harm.