For the sake of having the collection thus far in one place, I've taken the liberty of posting that recent addition to the Time Machine site. Credit, of course, belongs to Bob, thanks to the courtesy of the Directorate of the Catherine Palace Museum.
This leaves just 3 watercolors to be found to complete the full set of 12!
Thank you for posting that Sarushka! I do hope we can locate the last three. I'm surprised that there was only one of Alexei's bedroom and none so far of the Crimson Room and Grand Duchesses' Dining Room. Perhaps the last three include these rooms?