Here is a complete picture list with all the RARE dyes currently in the game, including a selection of uncommon ones that are hard to get. My main char was patient enough to pose for a screenshot of each dye
I made this post to help players who might be after a certain dye, but don't know exactly how it looks, or if it is worth the effort, because as you will see, many dyes are very similar.
I know this is not perfect, nor it can be, because how dyes look depends a lot on the costume/outfit.
So I picked "Drifting Sand" Tunic and Sash because of simplicity. It does not change the hue much, does not make the colors to dull or shiny, and has parts with different level of shine. The thin sash is the most shiny part of this costume, and it has the same shine like a metal pieces of armor. I used an outfit station placed inside, with an Alinor lantern nearby, to have a neutral to warm lightsource, for "shiny" colors. Screenshots were taken during game night time, so other lighting condition won't affect the setup (I don't know exactly to what degree the outfit station resets lighting, but wanted to be on the safe side). In the begining I wanted to post only material crops instead of showing my char, but I think it is better to see the whole costume in order to properly evaluate the dye.Rare dyes:
Akatosh Scales Gold
Grand Overlord's Brass
Grand Warlord's Violet
Magnus Setting Red
Motif Master Brown
Pact Blood Red
Putrid Flesh Teal
Ruby Throne Red
Stalhrim Chrome (Wolfhunter DLC)
Vekh's Mystic Blue
Uncommon "rare" dyes
Covenant Fisher Blue
Dominion Fisher Yellow
Pact Fisher Red
Shrike Blue (Summerset completionist)
From my experience, the most desired basic colors in this type of game are white, black, and metallic colors to dye weapons, armors, and other outfit elements. Shiny brass-gold is quite desired. Some of these are hard to get, other not so difficult. One big surprise is the Rank 5 material black, which is quite good for a very easy to obtain dye (deconstructing mats). Other very hard to get dyes are a let down, but you be the judge of that. From my experience, Liar's linen is great for silver lining, or steel-like weapons and metals, and Transliminal Violet the best for gold insertions.
Julianos White remains the brightest best white, but Palatine's White is also great (Julianos has a slight blue tint, Palatine's has a warm pink tint).
Below is a comparison chart for these dyes. I might update it with crops taken from a shield or armor for metallic colors.
I hope you will find this useful, and if you have ideas how to make this better, please comment below.