The main RuneScape Wiki has an article on: Bluedye
The Old School RuneScape Wiki also has an article on Bluedye.

Bluedye[sic] is an item which can be made by giving Aggie in Draynor Village 5 coins and 2 woad leaves. This item can be used to change the colour of a cape, giving the player 2.5 Crafting experience, or used with Goblin Armour for the Goblin Diplomacy quest. The dye be also used with Yellowdye to make Greendye or alternatively the dye can be mixed with Reddye to make Purpledye.

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