I hope this is the right place to post this.

I was trying to colorize some line art and decided to try using the interactive tool in gmic, however when I click apply I get an error thrown that says,

***Error in ./fx_colorize_interactive/*repeat/*substitue/*** Command '-x_colorize': No Display Available.

Is this a bug or am I doing something wrong?

First question: _which operating system?_

Then, I got another report like that, and the reason for that should be that we built with the wrong display option for cimg, but I'm pretty sure those are set correctly in the build system. I'll have to verify before doing a 3.2.1 build end of this week, but I tested some filters that would pop up extra windows other than the basic gmic-qt window, and they worked for me...

Windows 10.

You are right
I have Krita 3.2.1 and the same problem (windows 10)
The problem is only with colorize (interactive) - only this I noticed.
The solution is to download latest version GMIC filters from http://gmic.eu/files/prerelease/, put in place when you can found it and open Krita.
Next choice - settings -> configure Krita -> Gmic QT integration - and change path to this Gmic filters version what you just downloaded. For example in my cases is : C:/gmic-krita-qt-2.0.3-win64/gmic_krita_qt.exe.
Reset Krita, and everything works fine.

This work in my solution I hope to solve your problem too

I tried that now gmic wont even appear
What is the default path for gmic?
Incidentally I installed the latest version of Krista before I tried the above suggestion and I found the Graphic novel and another filter I liked disappeared.

Ok I got gmic working but I'm still missing two filters, the graphic novel filter and repair scanned document.
Any help on that?

On the bottom of the list of filter, you have a "Update filter" button.
Graphic Novel is in "artistic" category. Repair scanned document is in "Repair". I have both of them in Gmic-qt for Krita 2.0.4 ( with Krita 3.2.1 ).

Thank You