Have to attend a colleague's wedding dinner buffet at one of the restaurants by Keppel Bay, we are not really close. How much should I give for the red packet?

If you are just there to fill the table, just give a nominal amount.

some people are just inviting to be polite, kindly reject it.

One of my pet peeves is the expectation that someone who goes to a wedding (especially reluctantly) is expected to cover the price of the wedding costs per head. And it's even more annoying if the couple is like this:

I would just give whatever amount you are comfortable with, especially if you feel that you are going to the wedding just to 'fill the table'.

I have had acquaintances reach out to me many years after not keeping in touch, and it is clear that they are looking to fill the table with classmates. Needless to say, I usually do not respond to such requests.