DTG released update for Class 377 so you are now able to use it in livery editor... but thats only half feature. Im sure you would like to even use your livery but i guess they dont think about that. So here is quick fix which will enable your liveries in timetable mode and in scenario planner for Class 377. Enjoy. http://www.trainsim.cz/?mod=article&showid=2435
It’s a bit silly but we’ve gotta remember they are converting the preserved trains to the livery editor for free so we shouldn’t moan too much. I’m sure they’ll sort it soon like the Talent 2 when that went missing.