UL doesn’t come with any apps, the green and white one is the stock one. But the default sms solution is kind of weird, you can see one of the posts about it here:
Sending messages in the “SMS” app seems to be no problem, but if you clicked on a notification saying you got a sms and wanted to answer it, it might have opened in the “Messaging” app instead, and if I recall correctly, it is difficult (if not impossible) to send messages in that one.
Anyways, it might be a good idea to download another messaging app to use instead