Punctuation after "P.S."

Solution 1:

There are some ways I've seen:

PS.
PS:

But they are "modifications" (i.e. postscript) that changed the original formula Post Scriptum.

It should be written as P.S. and after it you should just continue writing normally, such as:

P.S. I forgot to say that [...]

Solution 2:

I believe the colon makes sense because the designation of a postscript is not actually a complete thought. Rather, the use of "post scriptum" indicates that something will be forthcoming, which I would set off from the "P.S." abbreviation using the colon.

However, American and British English have both largely moved away from using periods in abbreviations (FBI versus F.B.I., for instance). However, I would still use the colon to set off the rest of the sentence:

PS: Please note that this means we will not meet on Monday, but on Tuesday instead.

But I must admit that I have a fondness for "P.S.:" as well.