The Modus themes are designed with restraint, so that their default looks do not overdo it with the application of color.
This is the non-quantifiable aspect of the themes’ design: the artistic part, if you will. There are a lot of cases where color can be used inconsiderately, without accounting for layout, typographic, or other properties of the presentation. For example, two headings with distinct markers, such as leading asterisks in Org buffers, do not have to have highly contrasting hues between them in order to be told apart: the added element of contrast in hueness does not contribute significantly more to the distinction between the headings than colors whose hues are relatively closer to each other in the color space.
Exaggerations can be hard to anticipate or identify. Multiple shades of blue and magenta in the same context may not seem optimal: one might think that it would be better to use highly contrasting hues to ensure that all colors stand out, such as by placing blue next to yellow, next to magenta, and green. That would, however, be a case of design for its own sake; a case where color is being applied without consideration of its end results in the given context. Too many contrasting hues in close proximity force an erratic rate to how the eye jumps from one piece of text to the next. Whereas multiple shades of, say, blue and magenta can suffice to tell things apart and avoid excess coloration: a harmonious rhythm.