Color is extremely useful in design for things like conveying mood, emotions or to add visual emphasis to through contrast to a visual design, by using things like complementary or analogous colors a designer can create high contrast or shading. In my example from The Best American Comics by Harvey Pekar the artist uses color in very cool colors, pretty much black and white, to show that the event most likely happened at nighttime in the past. It also sets the mood by making these frames seem very somber and depressing which fits because these scenes cover heavy topics.
When the graphic novel returns to color, it is implied that the following frames take place in present day by contrasting it to the black and white images before it, color also helps to convey the message that the guy closest in the frame is in prison by giving him an orange jumpsuit to wear, which is usually closely associated with prison garbs. In the final scene color shows just how painful and intense the electric chair was by making the person in the chairs skin uncharacteristically red and adding hot colors like yellow, orange to show the fire and electrical current passing through this man body. By using color for the reading section of the comics in yellow, it helps to give contrast to the cartoon portion of the comic and lets the reader know that these are two separate elements within the frame.