That's not a colon. It's a smiley face. Only it's missing something, so now it's just a sad smiley face.
I can’t tell you how many times I’ve been reading an article and encountered incomplete parenthetical punctuation. I see the opening parenthesis, my mind automatically ‘nests’ the information that follows (bookmarks where the road forks,) and next thing I know I’m back in the main subject, only I don’t know it’s the main subject, so I’ve lost the thread of the whole thing. How did I get here? Where did the aside end? How do I get back?
You can’t just leave the gate open for side comments to run off like wild tangents. You’re not a lazy writer, content with using ‘u’ instead of ‘you’ and ‘i’ instead of ‘I’. You were discerning enough to know you needed the brackets, so use them. Both of them. Close your parentheses.
Do it for the smiley faces.