how to give someone colored leather armor with command blocks
/replaceitem entity @p armor.chest minecraft:leather_chestplate{display:{color:10040115}}
This adds a red leather tunic to the player that is closest to the command block.
Edit: It seems Mojang has replaced the command in 1.17, its now /item replace entity @p armor.chest with minecraft:leather_chestplate{display:{color:10040115}}