It's a matter of individual tastes and preferences, but always like to undercoat in a dark brown as black can look too stark for me. Then I can dry brush flesh tones over the face and get details, and depending on how dry my brush is elsewhere I can highlight details as desired and the figures can take on a genuine dirty field-worn look.
For ACW Confederates doing a straight gray would work okay for gaming purposes I suppose, but actual rebel troops wouldn't be so uniform - even the grays of their uniforms would be different tones because of supply issues and wear and tear. Then there were the "butternuts" because of gray dye shortages not too far into the war uniform makers resorted to boiling tobacco leaves, and various nut shells to get a brown dye called "butternut." A Johnny might have a fairly serviceable uniform but the colors might have varied in a unit because of that. And of course, rebel soldiers often wore items from home and weren't above "appropriating" equipment from their Northern foes.
In the Valley Campaign of 1862, the Union political general Nathaniel Banks was soundly sent packing by Stonewall Jackson's smaller Army of the Valley, losing a large supply depot. Stonewall's troops, a bit ragged after weeks and months of campaigning gladly grabbed up captured Yankee uniforms and put them on. Orders were given for the troops to wear them inside out to avoid being confused with the enemy and friendly fire incidents until there was time to bleach them out to look more Confederate.
Definitely makes painting Confederate troops a bit more entertaining, if not more time consuming.
On the other hand, some prefer painting rebels BECAUSE of the variety and think painting Northern troops are boring because of their better supply chain. I kind of enjoy the "break" myself - but one can have fun with weathering and fading of the Union blue - and Sherman's troops weren't much for uniformity, either.
I am definitely looking forward to the new sets particularly with the (IMHO) spectacular improvement in sculpting - kudos! To think, I was half-tempted t back out of collecting 1/72 scale due to my eyes. But with your new Nappies and now ACW...