Kind of works

When my secondary display is set to sRGB (which is best for color accuracy), it will not allow me to adjust the brightness, so I thought this app would allow me to adjust the brightness of it independently of my MBP. To this point, using the keyboard brightness keys would only affect the MBP, so I was hopeful this could be a solution. However, although adjusting the slider does indeed affect the brightness of the secondary display, it also affects the MBP. So when I bring the brightness down to a good level on the secondary display, it makes my MBP too dark to use. And when I try to adjust the brightnes of the MBP with the keys on the keyboard, my secondary display becomes bright again. It’s unfortunate that the brightnes of each display cannot be adjusted independently.

swervilicious about Brightness Slider, v1.2.6