I concur with Antonio there are no "Tercio / regimental" flags - each company had a flag. Not sure if this will be of use but, some Tercios were permanent (e.g 1643 tercio viejo de los estados de Brabante (Count of Vilalba), tercio viejo de los Estados de Flandes (Count of Garcies), tercio viejo de los estados de Holanda (Count of Albuquerque / Baltasar Mercader), tercio fijo de Napoles (Prince d'Ascoli), tercio fijo de Lombardia (Antonio de Velandia), tercio fijo de Sicilia (Francisco de Castilia), tercio de Saboya (Vincent Monsoriu), and some based upon infantry of nations, i.e not purely Spanish tercios, raised in territories dependent on the Spanish Crown: Flanders, Burgundy, Sicily, Naples, and Lombardy.
From around 1621 onwards, and certainly in the last quarter of the 17th century, tercios would have started to look a little bit uniform in colour of cloth, as the royal & military administrate would contract civilian for supplies of cloth, e.g 25 May 1643 contractor to supply 5720 complete suits of clothing for soldiers, each consisting of cassock, pair of breeches, doublet, leather jacket, two shirts with collars, pair of hose, shoes, hat and sword.
There is the recorded first uniformed unit(s) in this period but it was not a Tercio and not the NMA.