Which pair of colors is not assigned numerical color units?

• A. pink and red
• B. blue and red
• C. greenish-blue and red
• D. blue and gray

Answer: (B)

The main idea is how color in water is quantified using a standard color scale. Numerical color units come from the platinum–cobalt (cu) color scale, which covers specific hues and intensities used in water testing. Not every visible color has a cu value—only those that match the standard scale receive a numeric color unit.

The pair that consists of blue and red falls outside that standard scale, so neither color has a numerical color unit assigned. In contrast, the other color pairs include colors that do align with colors represented on the standard cu scale, so they can be given color units. That makes the blue and red combination the one that isn’t assigned numerical color units.