I’ve been struggling with this too. The video preview from aftereffects through blackmagic multibridge pro (hdsdi to jvc monitor) is brighter than the movies it renders (which i play back with blackmagics deck control to the same monitor).. It is consistent when I render blackmagic 10bit rgb, but inconsistent with yuv 10bit or yuv 8bit.
I noticed that all of the blackmagic mode options in the video preview prefs in after effects are RGB.. I wish blackmagic had a YUV preview mode – maybe that would solve it?
Josh