Which factor could result in inaccurate measurements during colorimetric assays?

In colorimetric assays, the measurement of light absorption is critical, as it corresponds to the concentration of a substance in a solution. If precipitates remain in the samples that are to be analyzed, they can scatter the light or absorb it themselves, leading to errors in the measurement. This is because the presence of solid particles alters the path the light travels through the solution, resulting in inaccurate readings that do not reflect the true concentration of the colored solution being measured.

In contrast, the other factors may not introduce inaccuracies to the same degree. For instance, using an excess of reagents could potentially lead to saturation but doesn't directly affect the light measurement as much as undissolved solids do. Utilizing distilled water for dilution is typically standard practice in creating consistent solutions, and while handling samples under bright light may lead to photodegradation, it doesn't directly interfere with the accuracy of measurements like precipitates do.