Muahahahaha! :D Basically 3D-Sketchup Models. Some modified by myself (like... Dereks Motel room. Took me AGES to create so much chaos), some just downloaded and used how they are. The trick is basically to save two elements seperately: The outlines and the flat colors without lines. One can use
the image Sketchup gives u per default or install an extension like Ambient occlusion to render the scene (that's what I do). Then take the rendered image, put it into a program named "watercolor studio" and filter it, so it looks like it's painted by hand. Take the filtered image and the lines, go
to Clip Studio Paint and throw it all together. Add some color corrections, gradiant maps, etc... and finished :3 Sounds complicated, but I guess the whole process takes me 5-7 minutes per background. I use a lot of auto-actions and templates to get it done in a timely manner.
With gogos House i actually thought it was a liquified foto XD But yes, thanx for the nice answer. I already did a only BG Lineart to save time (cause it's really annoying, to always redraw it) and was thinking to put some colour to it. but yeah. Putting also colour to it makes totally sense! <3
Your guess was pretty close, because a render is .... basically something that resembles a photo a bit XD COLOR is always a good addition! *u* So do it!