This is not a functional data pack and serves only as a blank template to add new Armor Trims to the game.
Both the resource and data pack are necessary for new Armor Trims to function, I have pulled out and included only the relevant documents needed to add extra trims. Armor trim in this nature can be used to add new armor or reskin mobs in a way accessibly competitive with the current retexturing of leather armor dyes. This is not necessarily a tutorial but a collection of findings.

In its initial state armor can be tested by using /give @s minecraft:[​insert_armor]{Trim:{material:"minecraft:material",pattern:"minecraft:trim"}}

Data Pack Info:
trim_material [​Folder] -
Allow the addition of new material subtypes

"assest_name":"material"
material can be change to refer to a new type

"color":"#9A5CC6"
#9A5CC6 can be changed to alter text color

"translate": "trim_material.minecraft.material"
.material can be changed to use a different message

"ingredient":"minecraft:barrier"
minecraft:barrier can be changed to enable the player access to the material type (barriers are unobtainable in survival)

"item_model_index": 0.00001
Must be a decimal between 0.0 & 1.0 to create a new inventory icon variant (from testing its rounded to 2 decimals meaning there can be 9 new trims between each existing. 0.01->0.09, 0.11->0.19, 0.21->0.29 ect)

(Try to keep naming uniform i.e all uses of material should be changed together with the exception of trim_material)

trim_pattern [​Folder] -
Allow the addition of new pattern subtypes

"assest_id":"trim"
trim can be changed to refer to a new type

"translate": "trim_pattern.minecraft.trim"
.trim can be changed to use a different message

"template_item":"minecraft:barrier"
minecraft:barrier can be changed to enable the player access to the trim pattern (barriers are unobtainable in survival)

(Try to keep naming uniform i.e all uses of trim should be changed together with the exception of trim_pattern)

Resource Pack Info
atlases [​Folder] -
"trims/models/armor/trim"
"trims/models/armor/trim_leggings"
defines models for armor

"material": "trims/color_palettes/material"
defines trim color selection linked to new material

lang [​Folder] -
"trim_material.minecraft.material": "Example Material"
defines tooltip linked to material

"trim_pattern.minecraft.trim": "Example Armor Trim"
defines tooltip linked to trim

models [​Folder] -
acts similar to CustomModelData using "trim_type" as predicate
every armor/trim requires a model, base armor in normal folder & custom trims in customtrims

textures [​Folder] -
trim->model->armor
modifiable color coded textures to show editable areas

trim->color_palettes
color palettes used to fill in armor trims
Note: the base trim_palette are the colors needed to create gradient trims. Colors are swapped 1 for 1 from the trim_palette with the corresponding color from the new palette. IF YOU USE ANY OTHER COLORS IN THE TEXTURE DESIGN THEY WILL NOT BE RE-COLORED. The palette is included in the top corner of the textures for ease of use.

item->trimicons
icon overlays for custom trims