When I open my former obj file, they sometimes come up with the paint I made, sometimes blank. How to control that?
Only OBJs exported from ZBrush will have polypaint. (It’s a custom tag introduced by Pixologic.) If the model has polypaint it’s included in the file automatically. Only the most recent versions of ZBrush (2018 and 2019) can read the polypaint when importing the OBJ. A few other programs, such as XNormal, can read the polypaint from a ZBrush OBJ.
Is there a option to prevent zBrush to load the polypaint?
No, I don’t think so. But all you need to do is press Color>Fill Object with white color selected.