If you've ever visited the Istrian Peninsula, you probably noticed that the soil there changes drastically, from very red to quite non-descript bright-greyish colour. According to the geological and geomorphic structure, the Istrian peninsula can be divided in three completely different areas. The hilly northern and north-eastern part of the peninsula is also known as White Istria (Bijela Istra in Croatian; that part of the peninsula is quite uninteresting in the winemaking sense, since there aren't much, if any, grapes grown there). Istrian inland is mostly lower flisch mountainous tracts consisting of fertile marl, clay, and sandstone, which is why this part is called Grey Istria (Siva Istra). Red Istria is the region along the coastline, covered with read (mostly limestone) soil is called Red Istria (Crvena Istra in Croatian; Terra Rossa in Italian).
Since the composition of the soil is quite different, the wines coming from Grey Istria and Red Istria are quite different. That is why we decided to divide the Istrian peninsula into two wine roads, based on the soil of their vineyarsds. This division is not definite, for various reasons, mainly because many Istrian winemakers have vineyards on both red and grey soil. Additionally, while the border between the red and grey soil in Istria is something that is visible to the naked eye, on the maps we used to help us make this wine roads for you it's not 100% clear on which side some of the bordering wineries should be placed.