Hi Franco,
Natural latex can vary in color but this or minor imperfections or darker spots that are normal for natural latex doesn’t make any difference.
The 100% natural Dunlop latex cores that are made by different manufacturers and that come from different countries are all closely comparable
While there are a number of “safety” certifications for latex (the most common being Oeko-Tex and Eco-Institut) … there is only one organic certification for latex which is GOLS (Global Organic Latex Standard) there is more about organic latex in post #2 here and the posts it links to.
There is more about the pros and cons of a single 6" core vs two 3" layers in post #2 here.
Phoenix