Understanding the argument -
When teaching art students about the use of color, teachers should use colored paper rather than paint in their demonstrations. - Conclusion.
Colored paper is preferable because it readily permits repeated use of exactly the same color in different compositions, which allows for a precise comparison of that color’s impact in varying contexts. - Premise. Basically, it's saying that the color paper is better (than the paint) as we can readily use it in different compositions, and this standard color allows us to understand the precise impact of the color in varying contexts. For example - a student wants to know the effect of the blue color in 5 different collages. If the color remains standard/the same, comparing the effect is easy. But say, as in the mixing of paint, sometimes you get light blue, which may look better, and in others, you may get dark blue, which may look better, but there is no comparison.
With paint, however, it is difficult to mix exactly the same color twice, and the varying textures of the applied paint can interfere with the pure effect of the color itself. - contrast with respect to the paint.
The argument looks like
Premise: Colored paper is better than paint because it allows for precisely comparing a color’s impact in varying contexts.
Assumption or missing premise?
Conclusion - Teachers should use colored paper rather than paint in their demonstrations
There has to be some link between "precisely comparing a color’s impact in varying contexts" and the conclusion "Teachers should use colored paper," which is what option D does.
Which one of the following is an assumption required by the argument?
(A) Two pieces of paper of exactly the same color will have the same effect in a given context, even if they are of different textures. - "textures is out of scope."
(B) A slight difference in the color of two pieces of paper is more difficult to notice than a similar difference in the color of two samples of paint. - this comparison is out of scope.
(C) Changing light conditions have less of an effect on the apparent color of a piece of paper than on the apparent color of a sample of paint. - this impact of the light and comparison is out of scope.
(D) Observing the impacts of colors across varying contexts helps students to learn about the use of color. - ok.
Premise: Colored paper is better than paint because it allows for precisely comparing a color’s impact in varying contexts.
Assumption or missing premise - Observing the impacts of colors across varying contexts helps students to learn about the use of color.
Conclusion - Teachers should use colored paper rather than paint in their demonstrations
(E) It is important that art students understand how the effects of using colored paper in various compositions differ from those of using paint in those compositions. - this difference is out of scope.