Where can I find specific dye colors?

All dyes are available at Myriam npc.

Dyes functionality was added directly to her interface in patch 2.4.3, removing dyes from vendors. Gold prices for dyes remained unchanged.

Dyes seem to be found per difficulty level, and are random within the the game as to what colors you find with what merchants

Each difficulty level will have new dye colors in addition to the dyes from the difficulty levels below it

Normal: Aquatic (Light Blue), Spring (Light Green), Tanner's (Brown), Winter (Light Grey)

Nightmare: Cardinal (Red), Royal (Light Purple), Desert, Ranger (Green), Autumn (Orange)

Hell: Lovely (Light Pink), Elegant (Dark Pink), Marine (Blue), Summer

Inferno: Gold, Inferno (Red), Abyssal (Black), Purity (White), Forrester (Green)

Edit

The only exception seems to be the Act I dye merchant in New Tristan. He doesn't seem to live by the same rules than the other Dye merchants do, although I haven't quite figured out what his rules are.

Normal: Green, Brown, and Light Grey only

Nightmare: Wasn't paying attention

Hell: Only had Normal + Nightmare colors until either Act II, or a specific point in Act I. I can't remember which, but do remember being disappointed that he didn't have new colors, and know I spent quite a bit of time in Act I and checked his colors many times

Inferno: Had two new colors the first time I saw him, so perhaps I just had bad luck in Hell?