Contemporary reconstruction. Flag of Moldavia in the 15th-16th centuries. Made after: Seal of en:Alexandru cel Bun (1400-1432) from the National Museum of History of Romania. [1], Image:AlexandruCelBunSeal.png, Image:StefanCelMareSeal.png. 
The first information regarding the color of the Moldavian flag (including elements) comes from a book of documents published in Krakow in 1533: The grand flag had a red field on which was nicely painted with gold the coat of arms of Moldavia (banderim magnum sericeum, coloris rubri, in quo arma terrae Moldaviae pulhre auro depicta erant).
Own work

Reference: This is a vectorized version of the image from https://www.crwflags.com/fotw/flags/ro-mold.html
Shtephan