Leather armor has a special tag that is a Decimal Value color code
{Item:{id:"minecraft:leather_helmet",Count:1b,tag:{display:{color:11546130}}}}
using the spawning code for angles and what equipment they have,
just give the leather armor a special rng decimal color code, doesnt have to be specials colors
basically a mix of rgb to decimal
you could probably have a random range script that pulls a number between
Hex 000000 to Hex FFFFFF
or
Dec 0 to Dec 16777215
Decimal is a little weird with its values tho so I would recommend just a hex rng script
get 3 individual values from 00 to FF then combine them in order of RGB (not convert to RGB just order)
then convert the full thing to decimal and use that
Mercerenies
commented
2 years ago 
Leather armor has a special tag that is a Decimal Value color code {Item:{id:"minecraft:leather_helmet",Count:1b,tag:{display:{color:11546130}}}}
using the spawning code for angles and what equipment they have, just give the leather armor a special rng decimal color code, doesnt have to be specials colors basically a mix of rgb to decimal
you could probably have a random range script that pulls a number between
Hex 000000 to Hex FFFFFF
or
Dec 0 to Dec 16777215
Decimal is a little weird with its values tho so I would recommend just a hex rng script
get 3 individual values from 00 to FF then combine them in order of RGB (not convert to RGB just order)
then convert the full thing to decimal and use that
so for example HEX B0 2E 12
DEC 11546130
NBT: {Item:{id:"minecraft:leather_helmet",Count:1b,tag:{display:{color:11546130}}}}
Boom thats just for a hat
do that 3 more times for unique color chestplate, leggings and boots
this should help I think https://papermc.io/javadocs/paper/1.13/org/bukkit/inventory/meta/LeatherArmorMeta.html#setColor-org.bukkit.Color- apparently you can use has codes? https://papermc.io/javadocs/paper/1.13/org/bukkit/Color.html#hashCode--