The default secondary color for the Y-Wing Fighter, like the Ixiyen, is white, which matches the "bare" hull color. Thus, the Y-Wing looks like it has only a one color design. As can be seen from these images, the default pattern (Texture 1) actually has a secondary color that covers most of the rest of the hull. Although this isn't true for all the Textures, the Y-Wing is one of the few Rebel ships where you can paint the hull, like the Hutt Fighters.
In order to make the secondary color visible, I have changed it to yellow.