I only have two calibers, but for the 6.8 it's FDE, and the .223 Wylde is black.
Now, they also have different hand guards, the 6.8 is on a S&W lower, and the .223 Wylde is on a PSA.
Also the .223 Wylde and my son's 5.56 gun both use the Lancer smoke mags, and the 6.8 uses PRI mags, so plastic vs steel, and it's easy to tell the mags apart in the dark.
Now, I think what the OP is getting at, is if something were to happen to him, or say he has kids that may not be as familiar, but know enough about guns, and are old enough, that they have access to them without having to ask, how would he deal with that.
To that, the caliber needs to be marked on the outside of the upper and mags clearly.
For the upper , that bolt release looked like the deal. For the mags, get a stencil set and just lay the appropriate numbers on the mag and spray it.