So me and a friend tried to use mods that were server based and they all worked for me but for my friend not so much.
The colors of everything would go inverted in some cases and his character statuses would go into insane numbers, into the thousands.
We had three client only mods, both of us had them installed.
As well as we have tried using server mods without the client ones enabled to see if that would do anything but to no avail.
Also needless to say all mods were up to date.
I'd like to stress how this is with only server based mods and how this is with any server based mod just not some random ones.
Connect to a modded server, as that is literally all my friend did.