Dyed Armor

Before the 0.7.4 Update November 11th, 2019, players were able to use dyes, of any color, to color any armor of their choice. Now that dyed armor became unobtainable, the price of still existing pieces is skyrocketing. Therefore, prices can be upwards of millions of coins for a single piece of the armor to billions of coins for a full set.

Dyeing armor is still possible, but only with Crystal and Fairy armor. Therefore, armor pieces with Crystal/Fairy armor colors are worth nothing, except the price of the armor itself.

NOTE: Even though armors like Yellow Shark Scale Armor are not dyed originally, they are still expensive as they were obtainable through normally crafting upon release of the armor, which is no longer obtainable.

Hex Codes
In order to check the color of an armor piece, you need to check the Hex Code. Using the hotkeys F3 + H you can enable the formatting code which makes Hex Codes visible (not relevant on versions 1.16.2+).

Here is a list of crystal armor hexcodes:

Here is a list of Fairy Armor Hex Codes:

Rarity
Some dyed armor pieces are more rare than others, this is solely due to the quantity of dyed armor pieces and colors.

To give you an impression, of originally dyed armor pieces, this is an estimate of dyed armor and how many pieces MAY exist (in alphabetical order):

Armor of Magma: 15,000

Angler Armor: 3,000

Bat Person Armor: 25

Blaze : 10

Cactus Armor: 50

Emerald : 580

Farm Suit: 300

Guardian Chestplate: 800

Lapis Armor: 8,000

Leaflet Armor: 75

Mushroom Armor: 800

Necron's Chestplate: 3

Obsidian Chestplate : 1,300

Old Dragon Armor: 2,600

Protector Dragon Armor: 2,200

Pumpkin Armor: 50

Speedster Armor: 1,200

Squid Boots: 75

Stereo : 300

Strong Dragon Armor: 5,800

Superior Dragon Armor: 1,450

Tarantula Armor: 150

Wise Dragon Armor: 2,200

Young Dragon Armor: 3,300

NOTE:

Green appears to be the most expensive color

While Mint, and Blue appear to be the more expensive colors.

But Yellow is very common, because a lot of players used to dye their armor to recreate a fake Superior Dragon Armor set.

Additionally colors that are very similar to the Fairy/Crystal armor sets are also less expensive.

Prices
The cheapest sets are Armor of Magma sets and most Dragon sets, except Superior Dragon Armor due to their availability. These sets can go for around 8-30 million coins.

The more "exotic" the color, the more people will pay for it. Example: if it's a Lime Green, or a Light Blue, full sets of certain armors, like Superior Dragon Armor, can go for hundreds of millions.

Fully-dyed 'exotic' Tarantula Armor sets can go for upwards of 1 billion coins

Example of rarity comparison: Green Young Dragon Armor is way more expensive because of the color, compared to Black or White Young Dragon Armor.