According to this diff (seen on this OTRS bug report on bugs.otrs.org), those are the pertinent rules that are now missing in this new version of the OTRS system:

tr.Incoming > td {
    background: #d4defc;
}

tr.Outgoing > td {
    background: #d3e5b5;
}

I think that the articles themselves were also colored, but I am not sure on this and maybe that the presentation with the "bubble speech" is enough...

Hey, I don't know if this helps a lot, but I built a quick GreaseMonkey script that should restore the colors: https://github.com/Matthewrbowker/otrs-colors/raw/master/otrs-colors.user.js . This should work short-term until we decide what to do with our OTRS install.

Can you please put or link this somewhere on OTRS wiki? I think there are some more Greasemonkey scripts, at lease I have one that is a bit useful.

In T263243#6475206, @Krd wrote:

Can you please put or link this somewhere on OTRS wiki? I think there are some more Greasemonkey scripts, at lease I have one that is a bit useful.

Gladly! I'll take care of that right now.

https://otrs-wiki.wikimedia.org/wiki/Greasemonkey_Scripts (Feel free to expand or rename) - I also ported to GitHub as opposed to the Paste, which was more maintainable.