Dirt. Not a uniform tint but the uneven, settled kind that collects in some places and leaves others clean.
Three layers stack up. The base is a soft noise field that decides where grime gathers, and it distorts itself: the field is used to push around where it reads from, which breaks the smooth blobs plain noise would give into something more irregular. It is also squared before use, which drags the middle values down and makes the dirt concentrate in patches instead of spreading thin everywhere.
On top come two layers of specks. The first is gated so that most positions are never candidates at all, then trimmed to leave hard-edged flecks. The second is a sparser scatter mixed in lightly.
The layers multiply rather than add, so a speck always darkens what it lands on. Nothing here ever brightens the picture, which is the right behaviour for dirt.