Aluminum Grafton pens do not patina. The anodized coating prevents oxidation and holds its color permanently.
The Brass Grafton Mini Twist ships with a protective coating that keeps the brass bright, and it is durable. Under normal daily carry it holds up for a long time, so your pen will look much as it did the day it arrived. If you want it to stay that way, it will.
Patina on the brass is a choice rather than something that happens to you. Some owners remove the coating deliberately so the bare metal can darken and take on the marks of being carried, and no two end up looking alike. That is a modification, not a defect, and it does not affect the lifetime guarantee on workmanship, materials, and production defects.