Rapid serial visual presentation (RSVP) is a gaze-independent paradigm in event-related potential (ERP)-based brain-computer interfaces (BCIs), offering a potential communication alternative for individuals with severe motor impairments. While RSVP-BCIs have shown promising results, questions remain about how specific aspects of visual stimulus design—particularly in symbol-based communication systems—can be optimized to improve usability and user experience. This study aims to investigate how different visual designs of pictogram stimuli affect classification performance and user experience in an RSVP-based ERP-BCI system. Three conditions were tested: (C1) pictograms without a circular background; (C2) each pictogram presented on a red circular background; (C3) each pictogram presented on a uniquely colored circular background. Nine able-bodied participants performed selection tasks under each condition. Objective performance metrics (accuracy) and subjective measures (recognition difficulty, ocular fatigue, and mental effort) were collected. The results show that condition C3, featuring heterogeneous colors, achieved significantly higher accuracy than C2 in certain trial intervals. Moreover, participants reported lower ocular fatigue in C3 compared to C2, and lower mental effort in C3 compared to both C1 and C2. These findings highlight the relevance of stimulus design in RSVP-based ERP-BCIs. Incorporating color heterogeneity may enhance system performance while reducing cognitive and visual load. Future work should validate these effects in target user populations and real-world scenarios.