Many, if not most, of the grenadiers in the David Morier (Royal Collection, 1750-1751) have "gray" hair, but some clearly have brown hair of the same hairstyle. I have not read anything that suggests hair powder or wigs would be mandatory, but I would think the "gray" would not be used under campaign conditions.
At the Battle of the Monongahela (1755), the two grenadier company (44th and 48th regiments) went into battle with just under 70 men per each company.
In a campaign involving a good number a regiments, it was common practice to peel off all the grenadiers from each battalion and lump all the grenadiers into a "grenadier battalion." Essentially, this gave the commanding officer an extra battalion size unit to maneuver.