I use Fluid Acrylics and like to mix custom colors that I use often and store them in my empty Fluid Acrylic bottles. Does mixing the paint diminish the shelf life of the paint in any way? (I don't add any mediums or additives to the paint.)
Mixtures should store well, however, the earliest sign of change is the mixture thickening. Also, some colors naturally have better stability than others do. Whites, Blacks, Phthalos, Pyrroles, Hansas all come to mind for long term fluid stability. Some Quinacridones, like Magenta and Red do much better than Violet. Earth colors commonly thicken up faster than other colors.

As you can imagine, the variation of pigment keeps us very busy in the lab, and we are constantly trying to improve this shelf life. Our goal for each product is about ten years or more of shelf life, when stored in normal room environments.

Blends do add another dimension to this, however, I would say as long as the containers are sealed and kept well, they should remain usable for many years.