Setting a label to a file with AppleScript

I'm trying to put a colored label on a file with AppleScript using the following code:

set theFile to "HDD:Path:to:the:file.ext"

tell application "Finder"
    set label of file theFile to 3
end tell

but when I run it in a terminal with osascript theScript.scpt I get the following error:

theScript.scpt:144:178: execution error: Finder got an error: Can't set label of document file "file.ext" of folder "the " of folder 开发者_JAVA百科"to" of folder "Path" of startup disk to 3. (-10006)

Any ideas why I'm having an issue and how can I make it work?

It should be coded as a label index not a label and it needs to be an alias:

set theFile to "HDD:Path:to:the:file.ext" as alias
tell application "Finder" to set label index of theFile to 3