The bug
Items made of iron do not appear to use a standard color palette. This is the same problem as MC-248611, but for iron instead of leather.
It is possible to partition iron products into roughly two different major palettes, which we will call the "shiny palette" and "dull palette", respectively. However, there is still a considerable amount of variation between these.
Items using the shiny palette include:
Iron ingots and nuggets
Iron helmets, chestplates, leggings and boots, both equipped and as items
Chain helmets, chestplates, leggings and boots, both equipped and as items
Iron horse armor, both equipped and as an item
Compasses
Buckets of all content types
Rails, powered rails, detector rails and activator rails
Minecarts, both in item and entity forms
Flint and Steel
Iron axes, pickaxes, shovels, hoes and swords
Hammer in the anvil UI
Pistons
Blocks of iron
Iron pressure plates
Iron doors and trapdoors
Iron bars
Iron golems
Items using the dull palette include:
Anvils
Cauldrons
Chains
Hanging signs
Hoppers
Lanterns
Tripwire hooks
Mob spawners
Smithing tables
Expected results
The palette used for iron would be consistent throughout the game.
Actual results
The colors used are not consistent.
In 1.20 Pre-Release 1.