We've re-added colormapping dialog in A5.1 (should be out today), we only moved command to open it directly into menu inside PV.
To those having problem with changing colormapping, dynamic colormapping works only when using global colormapping options, it won't change values inside active camera. If you want to change dynamic colormapping while you have active camera, you can check "Override camera colormapping" in render settings (you can do that also during rendering).
The reason for this behavior is that could be hard to determine where exactly should be colormapping modified when you change some value (e. g. when you change exposure, should it be stored in active camera? or should set colormapping override in render settings? etc).