Just like that, except for two things; first, tracks is a reserved word in the iTunes scripting dictionary, so replace it with something else (e.g. ts); second, you can't just convert a track to a string with t as string; you have to choose what information you want. For instance, if you want the name, you can use name of t as string; if you want more detail, you could do name of t & " - " & artist of t; etc. Note also that if you want a more complicated predicate, you can just tack it on to the whose clause; for instance, file tracks whose name contains "Green" and artist contains "Bird".
If you want to golf the script a little, you could replace it with the following:
tell application "iTunes"
repeat with t in (file tracks whose name contains "Green")
display dialog (name of t as string)
end repeat
end tell
Removing the of playlist "Library" might produce slightly different results, but probably not. It didn't in my quick testing.