Malak wields a unique lightsaber model, that is longer and bears a unique Hilt. I ported as much colors as possible and are now useable. You will need to download JCarter mod first if you wanna enjoy every colors displayed.

Vanilla colors : Blue, Green, Yellow, Purple, Red, Mantle of the force, Heart of the Guardian.
JCarter mod colors : Cyan, Orange, Pink, Viridian, Silver

The lightsabers are fully upgradeable, and you can switch from a standard to a long lightsaber with the proper crystal. They have the same properties as a lightsaber.

Optionally, you can make the weapon better with specific statistics : +2 damage, +1 defense, +2 blaster deflection, -1 dexterity

The crystals will only be buyable at stores, 2 crystals for each planet (1 vanilla, 1 from JC to even things out) :
- Dantooine Courtyard store
- Tatooine TwoHead store
- Manaan East Central store
- Korriban first weapon store
- Kashyyk Eli store

I did not bother much about introducing them into the game, i'll update this in due time.

If you do not wish to install the stores, you can get the items with the following cheats in the console menu

A video to explain how to enable and access cheats

Crystals

Blue = giveitem g_w_sbrcrstl30
Green = giveitem g_w_sbrcrstl31
Yellow = giveitem g_w_sbrcrstl32
Purple = giveitem g_w_sbrcrstl33
Red = giveitem g_w_sbrcrstl34
Mantle = giveitem g_w_sbrcrstl35
Heart = giveitem g_w_sbrcrstl36
Orange = giveitem g_w_sbrcrstl37
Cyan = giveitem g_w_sbrcrstl38
Pink = giveitem g_w_sbrcrstl39
Viridian = giveitem g_w_sbrcrstl40
Silver =  giveitem g_w_sbrcrstl41

Lightsabers

Blue = giveitem longlghtb
Green = giveitem longlghtgr
Yellow = giveitem longlghty
Purple = giveitem longlghtpu
Red = giveitem longlghtr
Mantle = giveitem longlghtt
Heart = giveitem longlghtg
Orange = giveitem longlghto
Cyan = giveitem longlghtcy
Pink = giveitem longlghtpi
Viridian = giveitem longlghtvi
Silver =  giveitem longlghtsi

Disclaimer

The mod should be compatible with anything, except for the merchants part. If you have any mods that edits the same merchants, there will be incompatibilities.

If you want to enjoy the additional colors, first download JCarter extra saber colors mod

There is a current problem with the mod : If you try to put the Hilt Crystal into a double bladed lightsaber, or into a short lightsaber, it will crash your game. I will work on a fix.

Installation

First, check if you have a upcrystals.2da file in your override folder, located in your game directory. If you do not, copy and paste my own file into your override folder. 

Then, run the patchers needed by order, Vanilla and JCarter. 

After that you can optionally modify the statistics.

Lastly, install the modified stores.

Credits :

The lightsabers colors and models are entirely extracted from Vanilla, so feel free to use it as a resource for your own mods.
The lightsabers models bearing JCarter colors are tied to the textures from his mod. Every colors displayed are entirely his work only and i do not own them.
Tutorial writers, Kotor discord channel, and multiple members from the modding development community for their continuous help and insights (Thor110, DarthParametric, Effixian, JcCarter, Lonewanderer, SithHolocron, Wizard, and more that i might have forgotten).

I thought i was the only one with this idea in mind, but there is actually another mod that does the same thing, gotta give credit where its due : Longsaber Crystal Mod

However, my mod is still relevant because : It does not require manual editing for the 2da file, It supports JCarter's colors and it also introduces them in game optionally (cheats are not mandatory).