Blue dye can be obtained from Aggie in Draynor Village or Ali the dyer in Pollnivneach in exchange for 2 woad leaves and 5 coins. It can also be purchased from Oronwen in Lletya or Guinevere in Prifddinas.
Aggie - OSRS Wiki
Ali the dyer - OSRS Wiki
Grud's Herblore Stall. - OSRS Wiki
Purple dye - OSRS Wiki
Hopespear's Will - OSRS Wiki
Graceful outfit - OSRS Wiki
Apprentice Cordelia - OSRS Wiki
Aggie - OSRS Wiki
Isafdar - OSRS Wiki
Green dye - OSRS Wiki