I am considering building a 260 AAR, which is nearly identical to the 257 AI that I already have, but how to keep them safley segregated??

Nickel brass? Headstamps??

Just curious if anyone has any creative ideas along those lines..

Put them into different containers that are well marked.
I have a similar situation with my 223 and 7mm TCU loads but for added convenience I use 5.56 brass for the 7mm TCU and commercial 223 brass for the 223.
The loaded cases are in separate containers that are well marked. The empty cases go back into the same box. There is little possibility of them ever getting mixed up.

When I used to shoot high power I got tired of people poaching my brass during matches since I was shooting a Garand and it flung brass all over.I took a magic marker and put a big wide ring around each brass about half way up. It stayed on and clear through many resizings and cleanings. You might mark 1 caliber or put different marks on both.

Heavy duty Freezer Bags. Rugged and have a zipper type closer device. You can write on them and come in all sizes. All food stores sell them.
Weather proof,water proof,dust proof and you have to open them by intention. Hard to put the wrong cases in them by accident.

The 260 AAR is the odd ball.

Find a recipe to dye the brass some outlandish color.

I use different color markers on the base. That way I can reload the same cases. Ya gotta remember what colors are what.

A detailed scheme using different colored sharpis for
(1) boxes
(2) primers
(3) writing (on the notes IN and On the bags or boxes)
(4) writing on the brass
(5) detailed load notes
The zip locks are a good idea. I have found writing on the bags wears/rubs off.
A note IN the bag is needed.
Writing on the brass has lasted ALOT longer than I expected.
I use plastic ammo boxes for most of my loads. I try to use the same color 20 & 50 round boxes for each cartridge .
Write the cartridge number on both boxes w/ the sharpie AND then cover the writing w/ clear tape. This helps keep the writing from rubbing/scratching off.
Goob luck.

I have both a .260 AAR and multiple .257's, including .257I's, but no 7x57 so I simply use 7x57 brass to form my .260 AAR brass. This was a simple solution for my ammo separation. I have also done color coding of case heads by markers, but the color wears off too quickly.

I have three 30-06 rifles

All headspace quite differently

I use Rem brass in one Win brass in another and Frontier in the last one

I have graciously received some ( old .35 & .366 BTs. No longer produced) by some AR members.Both have whiteish tips AS do NABs. My solution to be able to distinguish them. Color the BTs black on the tips AND primers .

When using different loads of the same case & caliber, I use colour codes with a Sharpie.

If you are using Nosler ballistic tips, it become even more simple!