Ensuring the stability and safety of steel storage racks is essential, particularly in terms of their ability to withstand applied loads over time. The columns in these rack structures are commonly produced through a cold roll-forming process, which offers high productivity and the capability to create open profiles, known as uprights. However, imperfections arising from the roll-forming process can adversely affect the buckling capacity of these open-section uprights. Stub column compression tests, as defined by the EN 15512 standard, are used to experimentally determine the buckling capacity. In addition, numerical methods are often employed to analyze the buckling capacity of columns. One of the simplest approaches to introduce geometric imperfections in structural finite element models is through the linear superposition of scaled eigenmodes obtained from an elastic buckling analysis. Although the type and magnitude of column imperfections (local, distortional, and global) are specified in international standards, the rules for combining different types of imperfections are unclear, often requiring designers to consider multiple possible combinations to identify the critical case. This procedure, however, may overly penalize the buckling capacity, as some of the considered imperfections are unlikely to occur simultaneously. The objective of this work is to determine the imperfection values considering predefined roll-forming errors and identify the combination of errors that most significantly affects the buckling capacity of open-section columns. To achieve this, the initial roll-forming errors were measured, and the amplitude of each error was determined. Then, a Finite Element Method (FEM) model was developed, incorporating the measured imperfections. The numerical results were validated against experimental data, and the effects of the roll-forming errors and the most critical combination were determined.