When visually comparing colors, why is D65 light source better to use than Illuminant C?

A D65 simulator isn't necessarily better than an illuminant C simulator. For evaluating fluorescent materials, D65 might be better since it has UV content that more closely represents real daylight. However, you would need to be sure that the D65 simulator you are using truly has that UV content. The best choice for visual evaluation is always the light source that most closely approximates the standard illuminant data used in your colorimetric computations. So if your colorimetry (XYZ, L*a*b*, etc.) is computed using illuminant D65, then view samples under a D65 simulator. If you only have an illuminant C simulator to view samples, then it would probably be best to do your colorimetry with illuminant C.  more