Perhaps just by a single wavelength LED that emits the frequency you want? Like these 450nm ones?
https://www.mouser.com/Optoelectroni...z0yt3kZ1yzs64u

The data sheet for the first one puts the spectrum 440 and 460nm. So that's a pretty tight spectrum and you can buy them at any 5nm increment from 275 - 750nm.