*5.3. Limitations*

We identified several limitations in our study. The proposed color code is based on the results of a semantic differential survey which should be extended by including more participants with diverse characteristics to make it more robust. For example, colors have different meanings and symbolize different concepts across cultures. Therefore, instead of proposing a fixed set of color-sound-scent pairs, we believe a selection should be tailor-made for the audience. Similarly, if fixed, some color-sound-scent pairs can cause incoherence when used for different artworks. Another limitation is the different needs of the audience. Some people can find the audio and scent stimuli bothersome if used over extended periods.

Regarding the number of test participants required for usability testing, problemdiscovery studies typically require between three and twenty participants. For comparative studies, such as the A/B test that compares two designs against each other to determine which one is better, group sizes from eight to 25 participants typically provide valid results [82]. For psychophysiological pain tests, a problem-discovery test, Lamontagne et al. [83] found that 82% of the total paint points experienced by 15 participants were experienced by 9 participants. Therefore, the number of usability test participants in this paper of 18 is considered sufficient. Greenwald et al. [84] performed an implicit association test on 32 psychology course students (13 males and 19 females). Therefore, in future work, we will further investigate scaled implicit association tests with more participants.

### **6. Conclusions and Future Work**

In this work, we presented a multi-sensory color code system that can represent up to 30 colors. It uses melodies to express each color's hue and scents to express the saturated, lightness (light-dark), and temperature (warm-cool) color dimensions. The scent selection and pairing were made through a semantic correspondence survey in collaboration with eighteen participants. We evaluated the color code system to determine if using a multisensory approach eases the effort to recognize the encoded colors and help improve the color identification compared to the commonly used uni-sensory method. The results from the evaluation sugges<sup>t</sup> that the multi-sensory approach does improve color identification, however not for everyone. The cause of this seems to be the extra cognitive effort and sensory overload experienced by some. In addition, we integrated the color code into a sensory substitution device prototype to determine if the color code could be more suitable and expressive when exploring visual art color content compared to a tactile graphics alternative. The results from this evaluation indicate that the multi-sensory-based prototype is more convenient and improves the confidence for visual artwork color content exploration. Moreover, the results also sugges<sup>t</sup> suitability for artwork exploration since the multi-sensory stimuli improve the experience and reaction to the artwork. In the future, we would like to expand the color code to include more color-audio-scent pairs to study the applicability across different styles of visual artworks. In this work, two non-visual sensory reproductions of artworks were used to evaluate the proposed color code system. As future work, experiments can be are carried out using more diverse styles of non-visual sensory reproductions to further support the results proposed in this paper. In addition, we believe it is relevant to explore the effect of the semantic incoherence that could happen from the color code usage and its influence on the artwork experience and the interpretation. While experiencing color is just a fraction of the art appreciation process, we believe our work contributes towards designing and studying accessible art appreciation frameworks for all.

**Supplementary Materials:** The following are available at https://www.mdpi.com/2079-9292/10/1 4/1696/s1.

**Author Contributions:** Conceptualization, methodology, J.-D.C.; formal analysis, data curation, L.C.Q., C.-H.L.; writing—original draft preparation, visualization, L.C.Q.; project administration, J.-D.C.; Investigation, L.C.Q., J.-D.C., C.-H.L.; Resources, writing—review and editing, supervision, funding acquisition, J.-D.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the 2018 Science Technology and Humanity Converging Research Program of the National Research Foundation of Korea (2018M3C1B6061353).

**Institutional Review Board Statement:** The study was conducted according to the guidelines of the Declaration of Helsinki, and approved by the Institutional Review Board (or Ethics Committee) of Sungkyunkwan University (protocol code: 2020-11-005-001, 22 February 2021).

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Data Availability Statement:** The data presented in this study are available within the article. **Acknowledgments:** We would like to thank all volunteers for their participation and the reviewers for their insights and suggestions.

**Conflicts of Interest:** The authors declare no conflict of interest.
