Which wood turns grey, and which turns black with time?
You don’t want to leave the wood unstained. When wood weathers “naturally”, it will be 1) uneven and 2) blotchy. 

1) Siding under an overhang will remain its natural color while siding with direct exposure to the sun and elements will fade quickly. 

2) Wood will fade and weather differently based on where each piece is cut from the tree. 

Most paint manufacturers make a “weathering” stain. It helps all siding to quickly weather uniformly. 

Wood should not be left untreated. Even wood siding near the ground will have water (and dirt) splash up on it and stain it. 

BTW, we use Western Red Cedar, White Cedar, or Redwood for siding. You can order it with 3 sides surfaced and the face “rough-sawn”. It comes in shiplap, bevel, and board and batt. 

Any wood left to the weather long enough, will eventually turn black without some protection (stain).