Celtic Plaid - Act 5 Side Quest
Start Location: Glauberg
Quest Location: Random Towns in previous Acts
Objective: Collect 3 Dye from previous Acts
Celtic Plaid is a Side Quest which begins by speaking with Bricia in Glauberg, the 3rd town you visit during Act 5. She asks you to collect 3 dyes from previous acts throughout the game. You'll need to bring her Greek Purple, Egyptian Gold and Asian Royal Blue. You can find each of these different colored dye on vendors from previous acts in the game.
The items you buy and give her do not need to have the same exact name, they just need to be the right color. As you can see in my screenshot below the item is called Purple Greek Outfit, which is different than the item Bricia asks for, Greek Purple.
Greek Purple: Buy from Merchants in Act 1
Egyptian Gold: Buy from Merchants in Act 2
Asian Royal Blue: Buy from Merchants in Act 3
Most commonly you'll find dye sold by General Goods vendors or the Arcane vendors who sell mostly spell caster items. Most of the time (unlike in the screenshot below) Blacksmiths do not sell any dye. Sometimes a vendor won't have the right color that you're looking for. If this happens use the Portal to travel somewhere else then port back and check again, the vendor will be reset. Once you have all 3 different colors return them to Bricia to complete the quest.