While all of these words are associated with color, reddish does not fit.  Reddish is only an adjective used to describe another color.  Violet, blue and green can be nouns as well, and describe specific colors.

Each of the related words here pertains to smoothness. Something “satiny” is like the silky fabric “satin.” Likewise, velvety is soft like its namesake fabric. When something is “unruffled” it is in good order and is calm (lacking any rough, uneven patches). This can be used metaphorically or—at times, at least—literally (e.g. unruffled fabric). When something is calloused, it is made rough—as when someone’s hands are covered with calluses because of hard manual labor.

Each of the related words pertain to something that is average. Even something “mediocre” is not necessarily bad—it is merely an average amount, nothing extremely good. To say that someone is “mediocre” is perhaps an insult, but it is in the sense that “he or she is not all that great,” not necessarily implying that he or she is bad or completely lacking talent. Something “maximal” is obviously quite different from something merely “middling” or “moderate” (etc.), for to be at the “maximum” is to be at the highest point.

The words that are related all pertain to being intelligent and insightful. To be “perspicacious” is to “see through” to the truth of some matter. (It comes from “per,” meaning “through,” and the Latin root for “to see”—as found in “spectator” and “spectacular”).  To be penetrating is not merely to push through a boundary; it likewise can mean “mentally penetrating”—seeing the truth of a matter, getting to “the heart of the matter.” Someone who is “acute” is keenly perceptive or “sharp” (in the sense of intelligence). Only “dull” is different from all of these words.

Each of these related words pertains to some form of loud speech or being outspoken. Something is presented stridently when it is done so in a harsh, forceful way. Someone is vociferous when they speak in a vehement or loud manner. Vocal can mean “outspoken” as in “a vocal supporter.” Even “forceful” is related to these other words, insofar as it pertains to being vehement. The only word that does not match these senses is “understated.”

All of the alike terms in this question merely express possibility, not certainty. Even the cases of “probable” and “likely” do not express complete lack of doubt that something will happen. Something probable is quite likely to happen, but a doubt remains; however, something “indubitable” is without a doubt and cannot be questioned. An “indubitable outcome” is one that is completely certain. While we might be able to come up with a better term, among the options provided, this is the best option provided.

The four related terms all pertain to being rich or abundant. “Verdant” describes a lush and green state of country land. It comes from Latin roots for “green,” which can be found in the French “vert”and Spanish “verde.” Even though we often think of “prolific” as applying more to human actions (as in “a prolific writer” who writes many books), the word first means “producing much fruit.” (This applies to a person, of course, for our writer above produces much “written fruit”).  Only the word “barren” is opposed to the other words, for it means “not producing much vegetation, fruit, or seeds.”

All of these words that are related pertain to relations in a family. While it is rather easy to see this in “parental,” the others might be a matter of vocabulary knowledge. Something “filial” pertains to a child. A filial relationship is a relationship of a child to a parent. A fraternal relationship is that between brothers. (A fraternity is a “brotherhood”). An “avuncular” relationship is the relationship of an uncle to nieces and nephews. The word “genitive” might be a bit confusing, for it might seem to be the same as “to generate” (as in “to give birth to”). Actually, the word is a matter of grammar indicating the possessive case. In languages like German and Latin, the genitive is used much like we use the –’s to denote that one person owns or is related to something else. (Still the sense of “relation” is not familial, so “genitive” is not related to the other words).

Each of the related words indicates a certain kind of physical “roominess.” That is, they pertain to some three-dimensional (or perhaps two-dimensional) size or measure that is large and open. The problem with the word “lengthened” is that it really only deals with an extension in a single dimension. Even if you lengthen a board of wood, you are referring to the act of making it longer in one direction. Because of this word “lengthened” does not indicate “roominess” as much as it does merely the act of making something longer.

All of the related words pertain to health in some way, even if they indicate lack of health. To be vigorous is to be strong and healthy, while to be frail and ailing is to be weak and / or sick. Only “tepid” is not related to this group of words, as it means “lukewarm” or “lacking excitement or enthusiasm.” While this describes a certain state of mind for a person, it is not directly related to health—not even mental health, for someone could be tepid merely because something really is not interesting.