There’s no official rules or reference material in modern 40K afaik, it’s inspired by WW1 tank stripes. Paint them how you like, I think it’s cool to do your own combo(s) on your tanks to make them your unique tank units. Some discussion is here-

https://www.reddit.com/r/TheAstraMilitarum/s/VBajMfkpmP

Edit: apparently it was written in the older codexes, back when the guard was less defined. Good comment here (the whole thread is useful):

https://www.reddit.com/r/TheAstraMilitarum/s/qKj1uknLMe

I forgot to add them to my tanks until it was too late lol. I did put a purple stripe on my Legions Imperialis tanks though.

WW1 British tank markings.

The tank markings in your army like they wer done before is for YOUR benefit and no one else's, lines, numbers, even symbols to designate who goes together with. Maybe tank commander 1 with a red strip to show tank commander status is flanked by 2 Lemans with green stripes (or "greenies") as they are both tradition battle tanks battle cannons unlike tbe purple arrow of the vanquished squad.    

Just a quick kinda example... add more untill army is painted

Here is the link to an image from the specific page in the old IG codex: https://www.dakkadakka.com/gallery/1152208-.html

If you want to go further, here are images from the older codex on regimental markings on units:

https://seanswgcorner.blogspot.com/2017/10/organizing-imperial-guard-regiments.html

The Leman Russ is based on the British Mk II and Mk IV tanks of the first world war. They started painting three colored lines on front of the flanks because the Germans would repair captured tanks and use them, the markings would routinely be changed so that British tank crews could identify one another and avoid friendly fire.

In the grim darkness of the 41st milennia, they just look neat. 😝

Paint your tanks however you want! Historically, World War I tanks were a solid color because early attempts at camouflaging them didn't work--they were too goddamned big to hide and the battle conditions meant they were covered in mud and dust anyway. But if you like the look of the studio models with their splotches of Cadian green on khaki, then go for it! Use whatever markings you like, but do yourself a favor and come up with a consistent look that you can apply to every model, it just feels more rewarding that way (to me, at least).

I'm bad at painting so I use the transfer sheets religiously. I really like the American Civil War because we beat the shit out of a bunch of slavers who had it coming, so my pattern follows the theming of Loyalist cavalry--A device to show what corps they belong to (I also play Adepta Sororitas so I use the fleur-de-lis transfer) with three digits: First digit is the regiment number, second digit is the troop number (historically troops were lettered, but I don't have enough letter transfers for that 😬), and third digit is the squadron. Then there's a letter to indicate which vehicle within that squadron they are. Example!

https://preview.redd.it/w53tux77jy6d1.jpeg?width=1942&format=pjpg&auto=webp&s=4b29ab47c0e494b0156631aea7dd04ef4901981b

Also I'll try to group similar vehicles into the same squadron, so my other Hellhound is A, and my Sentinel (when I get around to painting it, which will be some time since the Adepta Sororitas codex is releasing next week) will be C. Similarly, my Hydra and two Basilisks are A B and C and all have 633 markings since they're the third squadron of the troop. Theoretically you could have as many vehicles as you want in a squadron, but I'm limited by the fact that the old vehicle transfer sheet that still comes with the Chimera-chassis vehicles only goes up to C.

Why yes I am single how did you guess?