Am I sinful for not speaking to my colleague who speaks ill of me, even though I return his greeting to please Allah Almighty, and he greets everyone except me?

You did well in greeting the man and not reciprocating his ill-treatment with ill-treatment. Patience and responding to evil with good are desirable traits. Allah Almighty says: "Nor can a good deed and an evil deed be equal. Repel (evil) with that which is better..." There is no sin upon you, and your deeds will not be prevented from ascending if you are not abandoning or refusing reconciliation. Make sure to greet him at least every three days, for in the Hadith (definition): "It is not permissible for a believer to abandon a fellow believer for more than three days..." And if he does not return the greeting, then he has incurred sin.