Control opacity in BrushLabels

[younesslanda]

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