I have been experiencing the exact same problem when exporting from Motion 5. Color looks great in Motion, then significantly darker when the exported file is opened in MPEG Streamclip or uploaded on Vimeo. If I open the file in Quicktime, it looks exactly the same as it did within Motion.
One possible solution is to mess around with your display’s color profiles. For example, if I set my display to the HD 709-A color profile, what I see in Motion is almost identical to the exported file when viewed in MPEG Streamclip.