Ive been loading both using 223 dies and load data (same for 308/7.62) for a couple of three or four decades now, and never had any issues doing so.

Ive never seen 5.56 dies in the wild, and none of my manuals have data for it. I do have a copy of Lake City's load data from 1969, but never had the powders listed to load with, and even if I did, Id still be using the .223 dies, so theres that too.

If you want to keep things separate, you could see if you could find some nickel 223 brass. That wont come off for the life of the brass. Ive done that in the past with 30-06 when shooting matches so I could ID and recover my brass afterward.

Other than that, I also make the base of the round with a purple marker (most used black) and that made any cases that weren't nickel easy to ID.

 

I write on cases with a sharpie, when I’m trying new loads. Once the wind blew away the paper I had, identifying everything in an MTM box. Still had the ammo, just didn’t know which was which. So now I write on them. Wind hasn’t blown that away yet. It doesn’t come off in my tumbler (plain walnut shell), but I only tumble cases until they’re clean enough to load again.

 

I also write on cases with a sharpie but it definitely comes off over time and with usage

 

There are ways to "dye" brass. One easy way is to load different bullets.

 

I’ve switched almost everything to 223 over the past few years since I have guns that arent spec’d for 556 and I have guns that are so no need to worry what I’m loading into what. I’ve never seen 556 do what 223 won’t do anyway, at least for what I use it for. I know the biggest difference is supposed to be beyond 500 yards but that’s not for me anyway.

 

I load all my 233/5.56 to 223 spec regardless of what the case is headstamped. Never had an issue.

if You’re really worried about it, a sharpie will be the quickest/easiest way

 

The exterior dimensions of .223 and 5.56 brass are the same. 5.56 brass walls are thicker if my memory serves me correctly.
Added: It appears case walls are the same thickness with .223 and 5.56. Good link to review here: 223 -vs- 5.56: FACTS and MYTHS – Ultimate Reloader

 

Why don't you just not load any of your loads to max? The only thing that separates the .223 and 5.56 is maybe 200-300 fps. Keep your loads around 3,000 fps. There's no reason to load them that hot anyway.

 

This is exactly what I do. I've been loading ammo for over 40 years and very seldom have I used max powder charges.

 

Brownell's used to sell something called Brass Black. We used it on brass framed pistols for prop guns. It held up fairly good.

 

Dimensionally speaking, there is no difference in the brass, especially after you run it through a resizing die and trim it.

There could be a difference in thickness but that won't affect anything. Mixing the 2 will affect your accuracy just like shooting 2 different kinds of 223 brass.

Keep all your brass the same (lots or headstamps) for better consistency.