No need to make up or false praise. All of those who are around longer than you and the retiring college knows that you both don't have much work experience together. Keep it that way, keep it simple.
"Unfortunately we did not get much chance to work together, but they were surely an asset to the company and will be missed thoroughly. All the best for the next round of your life. :)"
Also, if you gather any information from any of the colleagues about them, mention that while using that in your speech, do not say that as your own, people will know you are faking, even if that's not your intent. Like
".... I got to know from some of our colleagues that they were vastly knowledgeable on subject X. I am also told about the contribution towards building department Y from scratch which I really appreciate. I'm sorry that we did not get more chance to work with them, many of us have so much to learn from them. [....]"