Lydian is the brightest sounding major mode while Mixolydian being the darkest. Similarly Dorian is the brightest sounding of all minor modes with Phrygian being the darkest. To know more about relative major and minor, click here.

If we take the key of C as an example, then we know the related minor of C is A minor. Similarly, the related minor for C Lydian is A Dorian and for C Mixolydian it is A Phrygian. So if you write a song in C Lydian, for a smooth modulation, you should move to A Dorian and vice versa. Understanding the tonal characteristic of all these modes will allow you to use them efficiently in song writing.