I have a 6.5 -06 and brass is made from 25-06, 270 ,etc, wondering how you mark your brass so as not to get mixed in with say 25-06 or 270 as I shoot both those calibers also. I don't worry about myself as much, but i'm getting older and don't want anyone to get into trouble later.

Well, your 6.5/06 ammunition won't chamber in your .25-06 (unless the gun has a grossly oversized neck area). And if you chamber it in your .270 and fire it the only thing you'll experience is low velocity and very poor accuracy. Just mark the boxes clearly with what's in them.

I too have had a 6.5-06 reloading for it using resized 25-06 and 30-06 cases. There is no real way of permanently marking cases to identify them as 6.5-06 i.e. stamping or marking them with the moniker 6.5-06.
I used a red permanent marker pen to run around and over the primer as well as around the neck-bullet junction, not that I used any of the other cartridges based on the 30-06 case. The red ink marking was more useful to me for identifying the sized 6.5-06 cases from the parent stock of 25-06 and 30-06 cases. I actually didn't bother running new 25-06 cases into the 6.5-06 die to open up the necks, the Sierra SBT 140gr 6.5cal bullets seated straight into 25-06 cases.

Re Stonecreek's post I did read years ago about a couple of Aussie brothers who reloaded and used 25-06 and 30-06 rifles where one ham fisted clot managed to chamber and fire a 30-06 round in the 25-06 rifle. I guess the 30cal bullet pushed back into the case when it hit the 25cal leade allowing the case neck to size down to 25cal. Apparently the rifle did blow, I can't recall what if any injuries there were.

thats what i'm doing right now.

I say that when you get to that stage of life, or are too careless to see the difference, time to stop shooting and take up knitting.
Keep them in separate containers.

two easy choices, one easy and fast

get an electric pencil and draw a vertical Line after the name of the round 270|, 30-06|

second, get 280 brass and only use for the brenneke

I also have a 6.5-06. i load for a buddy who has a 25-06 and a 270. I don't have a 30-06 so I used 30-06 brass. But then I found Quality Cartridge makes 6.5-06 brass and ordered 300 pieces. Its very good brass. 300 will last a long time

If you've got wildcats that use other head stamps, the ammo boxes need to be clearly marked. There's no two ways about that.
I have no 25-06, so that's the brass I use for said round. Another option might be to use nickel brass exclusively for one round or the other.
I have a similar conundrum currently. I have 3 30-06's and want to build an 8mm-06. Quite similar rounds, Hmm.. Ive thought about making it an improved 8mm just to give them a really clear distinction. Maybe I will take my own advice and use nickel brass only for the 8mm.

my OPINION -
I would never use nickel cases for something that I might be fireforming/loading hot - works AWESOME for things like 45 colt (i grew up with the Lone Ranger, so there's that) but if the brass requires anything but trimming or moderate loads, i've found nickel harder to work, more brittle, and, frankly, shows pressure signs faster -

not worried about myself, but when I pass getting someone in trouble, I have never shot it took it in toward payment for stock job. Best solution may be to sell it.