.example-panel-red.mat-select-panel {
  background: rgba(255, 0, 0, 0.5);
}

.example-panel-green.mat-select-panel {
  background: rgba(0, 255, 0, 0.5);
}

.example-panel-blue.mat-select-panel {
  background: rgba(0, 0, 255, 0.5);
}