Drillers · Board game rules
What happens when you run out of grey cubes in solo?
Quick answer
Minerals (including grey/silver cubes) aren't meant to be a limited resource—if you run out, just use a suitable substitute to represent them.
Details
The rulebook explicitly states that credits and minerals are not meant to be limited: "If you run out, use a suitable replacement." [p.18 — FAQ] This applies in both the multiplayer and solo game, since there's no separate solo-specific rule overriding it.
There is one related solo-specific mechanic worth noting: during an Action Turn, if the bot needs to collect a silver mineral as part of its priority list but there are none available on any open floor of the mine, "they take a silver mineral directly from the supply" instead. [p.15 — Solo Mode] This isn't quite the same as "running out of the supply" itself, but it's the closest official ruling involving silver cubes and scarcity in the solo game.
Watch out: Don't confuse "no silver on the open floors" (which sends the bot straight to the supply) with "no silver cubes left in the general supply" (which falls under the generic FAQ ruling to just substitute something else, like a token or a cube of a different color marked appropriately).