I would say
1 painting all Humbrol matt sand diluted with white spirit
2 let it dry
3 make a wash wih raw umber diluted with a lot of white spirit and sponge it direclty sweeping all over many time, the shadows are now in all recesses
4 let it dry
5 a mix of yellow ochre and titanium white with a tiny tiny amount of red ( result as wha you can see on the reportages about Palmyre )
6 drybrush , add tiny amount of white drybruhs, add ... and on and on ...., your drybrushing must be lighter and lighter in touch
6b if need be you can restart a second time point 2 to 6, if you thing there is not enough shadows ( making less drybrush point 6 )
7 let it dry
8 a last dry brush, almost white, almost nothing on the brush, clean on your finger many time ( until there is almost not colour left), and then apply very very lighlty on the edges; corner, hyeroglyphes,
see the column of my dioramas Atlantis, made that way, one without adding red to the mix, the other perhaps with a bit to much red
http://www.mirofsoft.com/quelques-modèles-finis/phoenix/