There is a general principle used to describe the intensity of a mode. The more flat notes there are in the formula of the mode, the darker it sounds. So the Locrian scale is the saddest/darkest mode, because it has 5 flats: 1 b2 b3 4 b5 b6 b7.
Then how can for example A Locrian sound dark and sad, but it has the same key signature as Bb major? Major keys are the same as the Ionian mode, and the Ionian mode has no sharps and flats in the formula: 1 2 3 4 5 6 7. So the major key sounds happy and bright. So if the tonic is A, it sounds sad, but if the tonic is Bb it suddenly sounds bright and happy? How is this possible ?
An easier example : C Major and A Minor.