I don't think light stone should be a problem. Necrons have personalities and dynasties now, so there's no reason why yours should have a taste in light stone.
Back when I still read Necron background, their stonework was smooth, perfectly cut and seamless. If you go with that, you'd best go with stone that has a natural pattern so you get an interesting texture on something otherwise plain and devoid of marks. Or metallic inserts with Necron language and symbols for contrast.
I'd avoid cobblestone. Doesn't seem like a Necron thing to me.
Dust (more than dirt) is always a thing on a proper Necron world. You could use that to accentuate larger stretches of stone, cover up roughly cut/uneven corners, mark the outward facing part of the building where dust would blow in to contrast from a more protected far interior. And the like.
Crystal formations are popular with Necrons and can be used to bring spots of color to the scenery, though they're naturally occurring and thus not directly part of the tomb. You could use power conduits to similar effect on tomb structures, though.
Broken(collapsed) parts and alcoves with broken Warriors (spares from your Doomsday Ark, for instance) won't go amiss.
Obelisks (pointy stone thingies, not the unit) are a great place to put tesla muzzles on so that an path lined by obelisks doesn't just look all pretty but serves a defensive function as well.
And you need a dead Mechanicus explorator of some sort. Possibly next to the aforementioned obelisks.
Ok, got carried away there a bit.