Control opacity in BrushLabels

**Is your feature request related to a problem? Please describe.**
I am working on a semantic segmentation project, and I find it hard to correct the mask region due to the high opacity of the colors generated by the brush. I tried RGBA format in the background inside Label, but it transforms it into gray color

**Describe the solution you'd like**
Add an argument in BrushLabels to control opacity, like `opacity=0.6`

**Describe alternatives you've considered**
tried RGBA format in the background inside Label, but it transforms it into gray color