A little GIF of the background for this pic in progress :D I'm trying to keep these simple, so the background is just implied.
Basically I found a ref close enough to what I was imagining and used that to select colours from. I laid down a SUPER rough background just focusing on the big details.
From there, blur the hell out of it.
After the blur I add some detail 'fixes' using a large soft airbrush until I'm happy with it.
In the end I wasn't too keen on how close the colours were to Lexa herself, so I added a faint blue overlay layer to shift the colours a bit. :)