The sentence can be partially interpreted through polysemy. Both "green" and "colorless" have figurative meanings, which allow "colorless" to be interpreted as "nondescript" and "green" as either "immature" or pertaining to environmental consciousness. The sentence can therefore be construed as "nondescript immature ideas have violent nightmares", a phrase with less oblique semantics. In particular, the phrase can have legitimate meaning too, if "green" is understood to mean "newly formed" and "sleep" can be used to figuratively express mental or verbal dormancy. "Furiously" remains problematic when applied to the verb "sleep", since "furiously" denotes "angrily", "violently", and "intensely energetically", meanings which are generally incompatible with sleep, dormancy, and unconscious agents typically construed as conscious ones, e.g. animals or humans, which truly "sleep".