A modified finite-difference frequency-domain method is used to analyze the one-dimensional periodic metallic gratings. The dispersion of metal and the boundary conditions at material interfaces are properly treated to circumvent the possible difficulty of divergence. In the visible light or infrared regime, the dielectric constant of a metal is described by Drude model. It is demonstrated that the effects of cavity modes and coupled modes can influence the transmittance and reflectance spectra of symmetric metallic grating structures. As a result from our simulation about the interaction of narrow-band coupled modes, an asymmetric metallic grating structure is designed as an electrode and a color filter for the white-light LED.