You also don’t want to do that; just leave the : in the translatable string. Concatenating translatable strings with other strings is generally a bad idea.
You probably want to add a context attribute, instead; or a translator notice, describing where the label is used.
…and I would just like to add another hit on the nail: yes, even the : at the end should be there and is relevant for translation. For example, in French you’d want to add some kind of space (ask the typo nerds) before the colon, whereas in English you wouldn’t.
So just leave the whole string alone, translators won’t complain they have to translate both “Something” and “Something:”, it’s normal and more often than not they’ll use a software that will even give a meaningful suggestion once they did one or the other.