*2.1. Materials*

All solvents used were HPLC grade. For all chromatographic studies as well as dye extraction, Millipore ultrapure water was used. Luteolin (C15H10O6), luteolin-7-Oglucoside (C21H20O11) and luteolin-3 ,7-di-Oglucoside (C27H30O16) analytical standards were purchased from Extrasynthese®. Potassium aluminium sulfate (AlK(SO4)2·12H2O), potassium bicarbonate (KHCO3), sodium borate (Na2B4O7·10H2O), calcium carbonate (CaCO3), and hydrated alumina (Al(OH)3), were purchased from Sigma-Aldrich®, while potassium carbonate (K2CO3) was purchased from Merck®. Gum arabic to prepare paint references was purchased in pieces from Kremer Pigmente®.

Flowering branches of *Reseda luteola* were collected in June 2020 by A. Clemente, from wild populations near Bucelas, north of Lisbon, Portugal (38◦54 21" N–9◦6 30" W). The plant material was spread in a tray and air-dried in the dark, in a ventilated area at 20 ◦C.
