Not necessary, because even if differently-textured paper can lead to different effects of the same color, we have no reason to think that teachers can’t just use paper with the same texture so as to study the pure effect of the color itself.

Not necessary, because the author’s reasoning isn’t based on the comparative difficulty of noticing slight differences in color. The reasoning is based on the fact that with paper, it’s easier to use the exact same color; but with paint, it’s more difficult.

Not necessary, because the author never compares the effects of light conditions on colored paper to the effects of light conditions on colored paint. In any case, even if the light conditions have a greater effect on the colored paper, students can still study the impact of color in different lighting contexts. That possibility doesn’t undermine the argument.

Necessary, because if it were not true — if observing the impacts of colors across varying contexts does NOT help students learn about the use of color — then the difference the author points to between colored paper and paint doesn’t provide a reason that teachers should use colored paper to teach about color. The author must assume that that difference is something that matters; that it’s desirable to have what colored paper allows if one is learning about color. If observing color in different contexts isn’t helpful for learning about color, then why should we try to use colored paper?

Not necessary, because the argument concerns what’s helpful for students learning about the use of color. So the point of bringing up paper and paint was to comment on how those mediums relate to color and what students can learn about color. But the different effects on composition from paper and paint is a separate issue.