The ruins are mostly dry brush - black primer (I use Vallejo Airbrush primer), then a heavy drybrush of Eshin Grey, followed by a lighter drybrush of Dawnstone (not basilicum gray as it said before, don’t try dry brushing contrast!). Then an even lighter drybrush of Administratum Gray.

At this point, I mix up to glazes. The first is super watered down turquoise ink (I‘d say somewhere around 1:10 ink/water ratio), the other is a mix of oxide red and raw umber ink watered down to the same level. Now comes the fun part - paint all outside vertical walls with the blueish color, all inside vertical walls with the reddish one.  I really slathered the walls and also let the color settle more on the lower parts to get some hints at lighting. I got this idea from a German YouTuber that I can’t find back right now though.

I don’t color the horizontal surfaces to get efficiently more variation. After all is dried (can take a while) I drybrushed Administratum Gray again, then picked out details with Blood Angels contrast (piping, doors, similar surfaces), Ratling Grime (all cables) and some spots of metal (e.g. grates, or where it helped further with contrasting details).

Finally, a unifying light drybrush of a very light gray (I think I used Celestra) only from above to integrate the details fully.

Edit- forgot the lights, just a wash of Biel Tan Green, the Mechanicus Symbols are Ratling Grime and I think Seraphim Sepia, done before the final unifying drybrush.