Because the army of Flanders relied heavily on local makers for clothing and equipment even the Spanish & italian troops came to resemble "every one else" And of course many regiments were Walloon & German troops & equipped in the area to begin with. The main diffrences would be seen in some clothing styles worn by the troops who came up from Italy along the Spanish tRoad though clothes wore out quickly and in the style of armour used.
There are some books in Spanish and at least one in French such as:
Pablo Martin Gomez "El Ejercito espanol en la Guerra de los 30 Anos" (Almena Ediciones, Madrid 2006 & 2009)
Pierre Picouet "Les Tercios Espagnols 1600-1660" (LRT Editions 2010)
Both however make heavy use of 'modern' artwork though "Les Tercios" also does have large size images of period military art from the collection in the Prado. The cost and limited acess to paintings were museums hold the copyrights to photos of course put a limit on how much use you can make of such works. Cheap it ain't..
Below is a selection of Flemish and Spanish artwork showing Spanish troops pre-war or in the 1620's
Spanish troops at the siege of Ostend by Vrancx (painted in 1610-1615 and showing the style of those years)

Spanish troops raiding a village (By Snayers from 1617)

Battle of Nieuport 1600, painted 1610-1620 by Vrancx, Spanish troops with red field signs such as sashes, Dutch with orange sashes
Spanish troops at Fleurus 1622:
As paintered by Snayers:

Defending the Ferme Chassart

Cavalry in action
As painted by Carducho (Spanish on the right beneath the oversize commanders on horseback)
