Playing NWN on Windows 10 right now, and i have updated the game to the critical patch. However, I am having issues with my cloak: it keeps disappearing, or frequently changes colors. I googled for the answer, but everything seemed related to installing the critical patch, which i've already done. Anyone have any idea of what to do?

The Nwn which is on GoG is already patched with the latest & best patch.
There is no need to install anything else.

Try uninstalling the game fully. Back-up your save files (in case something happens).
But before you do that, take the cloak off, save on a different slot.

That has to work. Maybe some files got broken. Re-download the game using Classical Downloader.

1. Select the game in your library
2. Select "More" (Its to the right from the GOG Galaxy)
3.Select "GoG Downloader Links"
4. Select the version of the language you would like.
5. Download

I hope this helps
gogwitcher300

I think cloaks were not always visible. This was implemented with a full public patch, way after the second expansion, that allowed people to use the new premium modules' content- horses and other assets.

One experiment you can try, since I'm guessing you are in a campaign module that was created before this patch, is to create or save a module using the toolset (nwtoolset.exe). Cloaks that are brought into this module, or bought/found in it, might behave differently, because a module saved with your advanced version of the game will use the full assets.

Another thing if you are fed up is to use an editor like Leto to set your cloak's appearance to 0 (non-appearing). But changing a character in a savegame in this way is tricky.