Translation:The rainbow will disappear in little time.
I know it can be frustrating but it is technically difficult to match natural phrases between languages in either direction and hence difficult to mark in a way that will catch every answer that might be considered correct or even better. The official solutions that are considered inappropriately forced and literal are nevertheless useful as they illustrate the guiding logic of the source language and at least one of the aims is to speak fluently in Romanian. I actually enjoy studying the literal translations as they help me to achieve that. Hopefully, I already have fluency in my mother tongue. Viva Duo!
I would always say "in a little time", not "in little time". And even then I would usually express it a different way.