Possible Clues:

• Ancient blue dye
• Sandwich containing egg -- and dye!
• Mediterranean plant that yields a blue dye
• What makes you blue and sorrow'd, say?
• Plant of the brassica family, an ancient source of blue dye
• Colourful herb and egg in a sandwich?
• Dye originally worn as decoration, hiding nothing
• Mediterranean plant yielding a blue dye
• Dye without notice
• Dye used by ancient Britons to paint their bodies