First off all, the hints for satisfying the ghosts with your paintings is hidden deep within the game files.
This was hinted at in the Halloween update 2017: https://cs2bus.com/games/582550/announcements/detail/2361435889197839694

This post, more specifically the bottom most paragraphs, give hints to how you can get the hats of the ghosts. The most important sentence however is this one:

Originally posted by Flamebaitgames:

Forgive me, I usually don't go off track like this. Most times when I try to es... I mean, when I try to infiltrate video game source files! Most times when I do that, I can focus much and ... THE FINAL LETTERS ARE "E" AND "S" *brrttzz*

The letters "E" and "S" refer to the "CES" folder in the game files. To access it, open your Passpartout folder by right-clicking on the game in you library, clicking "properties", going to "local files" and clicking "browse local files".
You will see something like this:

From here, click "Passpartout_Data" > "StreamingAssets" > "CES".
You will now see a lot of language folders which contain information about certain NPCs in the game.

Click on the "edit" folder.

Now you can see all the files referring to the characters in the game. These are pretty useful for finding out what every character likes, e.g. colors or color complexity.
However, that isn't important right now. What is important are the files beginning with "Ghost". There are 6 of them.

Now, to open them, you will need a text editor of some sort. You can use the basic Windows "Notepad", but for this guide I will be using Notepad++. It's much more compact and easier to read.
You can download it here[notepad-plus-plus.org].

Now, if you open them, you will see the painting criteria for satisfying them.
In the next few parts, I will explain what every file means and what painting you have to paint to get the hat.

When you open Ghost_Drumpf.lua, you will see this:
It may seem complicated, but it isn't. The green sentences are notes left from the devs.
"bid_dialogue" is the text that will appear when the ghost wants to buy your painting.
"function initialize" is the criteria for the ghost buying you painting.
"function onEvaluatePainting" is if the ghost will buy your painting, and for how much.

Now, the most important part is "function initialize". The devs have left a note as well for getting the hat of Drumpf.
"--It isn't huge."

To get his hat, make sure you make a painting with a few short strokes. This is what "score.brushLength" refers to.

Drumpf appears in the Aristocrat or Expressionist Act III.

Forenote: I haven't tested this, as I haven't seen Jack in the game yet. I also don't know in which act he appears in. If anyone knows, please tell me in the comments. :)
Apparently he appears on Halloween along with Robert!

Now to Jack. His file looks like this:

To get his hat, make a very "unoriginal" painting.
Sadly, I myself don't know how originality is calculated, but your best bet is to make a lot of very similar paintings in a row when he is in your level.

Forenote: I also know little about him... if you know anything, please tell me in the comments!
Robert's file looks like this:
Apparently he appears on Halloween along with Jack You can set your PC clock to October 31st, haven't tested it yet, though.

There are few new parts to this file.

The first part refers to the ID's of the colors in the game.
"function generateColorDominanceSumString(values)" is code to calculate which colors appear in a painting and at what frequency.

To get his hat, make a painting with every color on the right half of the palette. I have to test this myself as well, but it should work.

Shade's file looks like this:

To get his glasses, make a painting ONLY with the colors black, yellow and white. (White is just blank canvas)
You can just make a black canvas if you want to.

Shade appears in the Street Alley Act II.

Alright, now the next two will be getting complicated.
Stormcrow's file's first part contains ASCII art:
The second part contains this:
The art in the first part is a Lord of the Rings reference: It's the door of Moria. This door only open if one says the elvish word "mellon" to it. That is the first step to getting Stormcrow's wizard hat: Name your painting "mellon".
How you have to paint your painting, however: That is a bit more tricky. The dev's note reads:
"-- Count them all and say the word."
We already know the word, but "count them all" is a tad bit more tricky to find out.

What it means is that you have to have every color (except on white) on your painting, and these colors are only allowed to be one part. What I mean is that you are not allowed to have two unconnected "islands" of paint on you painting.

What you have to do is put 20 different colours on a canvas, with a singular island of each colour. Credits to caitlyn_bree for correcting me on this!

Here's a screenshot of me getting him to buy my painting:
Here's a closer look at the paintings:
Stormcrows appears in the Street Alley in Act II.

Alirght, last one. This is the most complex, as the devs say themselves in the files:
In this file, there are three messages written in binary.
The message under the first "addCriteria" translates to "Cynth".
The second one to "Benjamin".
These two messages refer to the Punk Cynth and the old man Benjamin, however I do not know why the devs put these names in here. I've looked at the both Benjamin's and Cynth's LUA file but I can't find a correlation. The only real thing they have in common is that the both like low color complexity and low originality.

That doesn't matter, however. The third binary message translates to:
"The birthyear of Passpartout. Search the book of faces." Because of this, we know that we have to name the painting a certain year, which I haven't found out yet. But you can still manage to get the painting by using a little loophole. Also, "Book of Faces" refers to Facebook, and I actually once sent the Flamebait Games' Facebook account a message:
Now, I don't know if this is actually the "birthyear" of Passpartout, because it didn't work when I tried it, but I may have done something wrong, I don't know.

Now, it seems there are no direct criteria for getting Voidwalker to buy your painting. Right? Well...
Remember the Halloween post from 2017? Well, in this message, there is a hint to further details for Voildwalker.
In the second part of the post, the ghosts speack to the reader, and Voidwalker says:

Originally posted by Voidwalker:

Forgive me, I usually don't go off track like this. Most times when I try to es... I mean, when I try to infiltrate video game source files!

Now, if you look at the very bottom of the CES folder, you will see a "void.lua" file.
Open it.
Aww man, I gues there's nothing here... or is there?
Scroll down a bit and you will see this:

Jackpot!

Alright, now I'll tell you how the criteria for satisfying his painting needs.
Make a completely pink canvas, and make it very quickly as well.
Now, here's the loophole: Name the painting "1111100001". This is the apparent "birthyear" of Passpartout, although it doesn't make any sense. But it works!
Like I said before, I don't know if "1928" works. It may, I'll have to do some testing.

Here's a picture of me getting him to buy my painting:
Voidwalker appears in the Minimalist or Expressionist Act III and gives you a funky hat.

Here's a quick overview of how to get every hat:

I hope you've enjoyed the guide! If you find any mistakes or have corrections, please tell me.
And if you know where certain ghosts are that I don't know, please tell me as well!
Also, this discussion from a few years ago helped me out, so credits to them:
https://cs2bus.com/app/582550/discussions/0/1480982338960135632/

If you enjoyed this guide, please give a thumbs up!
I'll be updating this guide with more pictures as time goes on, but for now: