Problem with text Opacity in a 3d scene

Hi all,

i'm creating a 3d scene (using viewport3d); the scene contains a collection of primitives (cylinder, cube...) and each primitive has a label to plot its name. I created the label with this code:drawing text in a 3d scene. I slightly modified this code to add also a background color to the label.

In the UI i have a slider to change the opacity of some elements (and so the opacity of the labels).

The problem is the following: if the foreground color (ie, the text color) is white, when i change the opacity the background color vanish out, but not the white text.

How can i solve this problem?

Thank you in advance!