It's down to rarity, plus supply and demand. The dye (and material) vendors in 'post searing' Tyria can only sell what players sell to them. Then them's the 'drop rate'. This is only a rule of thumb guide and based on researching the 'pre-searing' interlude or trading introduction to the game.
Basically I killed every hostile creature on an area by area basis and logged the results. It goes roughly like this:
* For every 150 hostile creature's item drop you might get one coloured dye (red, blue, green etc)
* For every 10 regular colours you might get one silver (less than 1 in 1,500 chance)
* For every 10 silver dyes you might get one black or white (less than 1 in 15,000 chance)
I could be scientific and spend months just 'farming dyes' to get a precise figure but it's the work of seconds for NCSoft to alter the drop rates and apply it to the next update. You may hear of people seeing two dyes one after the other but - especially where black is concerned - you are talking lottery number chances.
Here's a useful link: GameWiki on dye drop rates
What are dyes worth, I see them requested a lot ?
As a rough guide, for those those haven't visited dye vendor yet:
* Purple, orange and yellow cost around 100g
These vendors will only offer you 10g or so
* Green, red and blue, being more popular, are around 175g each
Vendors will offer in the region of 60 to 80g
* Silver, being both rare and popular, costs in the region of 750g
These can be sold to the NPC traders for around 600g
* Black and White, the rarest, costs in the region of 8,000g, rising to up to over 12k in times of demand, White being the more expensive or the two.
Ever popular, these are worth over 6,000g if sold to the stalls
As an aside, the white dye sold for a staggering 32k when it first came out in October 2006
Typically prices with vary by 25% from day to day and (scammers aside*) there's a large number of people making money in the game just by trading these, especially given that black can vary between 6,000g and 10,000g over the course of a week. Naturally, if you spend the time, you can buy or sell for a better deal from other players.
Black/White dye con artists
In 'pre-searing' especially you will frequently see "WTB Black dye" or similar. If you pull them up there will claim there are really offering 5,000g. Naturally we all know they are lying, otherwise they would announce the offer price. As mentioned earlier, a black vial of dye costs as much as 10,000g (with highs of over 14k) in 'post searing' Tyria and Cantha, meaning the vendors will typically pay between 6,000g and 8,000g. These con artists - for that is all they are - typically offer new players 200g and laugh at how gullible these 'noobs' are.
Screen shot here, nm works better for this, a good thing to do is Jaya Bluffs Feather Farm in nm over and over...eventually youll se results. not to mention if you get a stack of scalps they sell for 40k becasue each scalp or crest contains 3-4 feathers. if you guys get the chance to check out the screen shot please re rate it to a positive:)
below i got a black dye on my 3rd run. Combine together with my Renew Farming Guide located here
CLICK HEREto get higher results...the polls don't lie.