Yes, the colon is a syntax error. But in Version 3.7.2+180508, adding this to 1 twig file this results in the 'Theme editor' link for all themes leading to a browser message that the server is redirecting the request for this address in a way that will never complete. Subsequent use of the browser back button leads to the following appropriate limesurvey message: Arguments must be separated by a comma. Unexpected token "punctuation" of value ":" ("punctuation" expected with value ",").

The behavior at demo.limesurvey.org (3.6.3) is a little different/better. Entering something like {{Test:Test}} or {{TOKEN:ATTRIBUTE_1}} there (yes, neither one something that you'd want to enter anyway) leads to inability to re-access the theme editor for that particular extended theme to correct the error.