When it comes to pairing pants with a navy blue shirt, there are several options that can complement the color nicely. Since you mentioned that the navy blue shirt is not plain, it would be helpful to know what other colors or patterns are present on the shirt. However, assuming the shirt has no conflicting colors, here are some general suggestions:

1. Khaki or beige pants: Navy blue and khaki/beige create a classic and timeless combination. These neutral tones balance each other well and create a sophisticated look.
2. Gray pants: Light gray or charcoal gray pants can be an excellent choice to wear with a navy blue shirt. This combination offers a sleek and polished appearance.
3. Olive or forest green pants: If you’re looking for a bolder and more adventurous pairing, olive or forest green pants can work well with navy blue. This combination creates a stylish and slightly unconventional look.