From my understanding, Brightness is a High frequency lifter / Volume, it work quite similar to "Exciter" parameters on vocaloid version 5 and 6. Keep in mind that some voicebank have different behavior when you increased the brightness (especially at very high amount), when you decreased the brightness param it work quite similar to Dynamics param (DYN params is just a fancy name of volumes).
The way to used Brightness param is to push some vocal part foward, make vocal feels up front or decreased the value to move vocal back, if you low the brightness a lot it make vocal sound like singing in the throat. The way i like to use it to make vibrato sound expressive by draw a Pitch param combine with Brightness param to create a expressive vibrato sound, just like the picture below.
As for the Breathiness, i don't really use it that much, maybe 2-10 from 127 at best. I use it to make vocal sound unclear, make it feel mellow as part of the vocal design, most of my song with miku have breathiness active very often.
Although some voicebank sound very brittle/broken when adding some amount of brightness into it. In my experience, vflower is probably one of the extreme example lol and some voicebank are thrive when i add some brightness like, Lumi, Luka.
I hope this help!
