I see. Lighter and darker shades of materials are used to create contrast. It is used as an environmental divider. Take a look at your reference to see how the different materials are used. The bricks acts as a wall trim and it is used to distinguish the wall from the floor. Bricks are mainly used to convey strong foundation which is why its in the lower part of the wall. The light shaded wood material on the other hand adds emphasis for the counter for the player to notice (since the player will be cooking in this game I think?). These materials act as clean breaks for the materials to avoid being to repetitive.
Like what I said about emphasis, you can also use the advantage of contrasting materials to communicate with the player. In this case, the wooden materials have been darkened to highlight the chairs, which are colored in lighter green tones to give emphasis to the sitting customers.
Visual communication plays a key role when it comes to using contrasting materials. It really depends on how you want it to use it. The first example used a light colored wooden material to highlight the counter. In the second example, the darker wooden material is used to emphasize the green chair where the customers are seated. Lastly, smooth plastic should always make up the majority compared to the other materials to keep the low poly look.