How to change color opacity

[Cave1212]

i have this code

 float scalingFactor = 1.0f / 2.111f;

        glEnable(GL_COLOR_MATERIAL);
        glShadeModel(GL_SMOOTH);
        double minValue = std::numeric_limits<double>::max();
        double maxValue = std::numeric_limits<double>::lowest();

        vtkSmartPointer<vtkColorTransferFunction> colorTransferFunction = vtkSmartPointer<vtkColorTransferFunction>::New();
        colorTransferFunction->AddRGBPoint(0.0, 0.0, 0.0, 1.0); // Blue for low values
        colorTransferFunction->AddRGBPoint(0.5, 1.0, 1.0, 0.0); // Yellow for medium values
        colorTransferFunction->AddRGBPoint(1.0, 1.0, 0.0, 0.0); // Red for high values
      
        vtkSmartPointer<vtkPolyDataMapper> mapper = vtkSmartPointer<vtkPolyDataMapper>::New();
        mapper->SetLookupTable(colorTransferFunction);

        vtkSmartPointer<vtkDataSet> data = vtkDataSet::SafeDownCast(reader->GetOutput());
        for (vtkIdType i = 0; i < data->GetNumberOfCells(); i++) {
            vtkCell* cell = data->GetCell(i);
            vtkPoints* points = cell->GetPoints();

            double scalar[3];
            points->GetPoint(0, scalar);
            double color1[3];
            colorTransferFunction->GetColor(scalar[0], color1);

            points->GetPoint(1, scalar);
            double color2[3];
            colorTransferFunction->GetColor(scalar[0], color2);

            points->GetPoint(2, scalar);
            double color3[3];
            colorTransferFunction->GetColor(scalar[0], color3);
            glBegin(GL_POLYGON);
            glColor3dv(color1);
            glVertex3f(points->GetPoint(0)[0] * scalingFactor, points->GetPoint(0)[1] * scalingFactor, points->GetPoint(0)[2] * scalingFactor);

            glColor3dv(color2);
            glVertex3f(points->GetPoint(1)[0] * scalingFactor, points->GetPoint(1)[1] * scalingFactor, points->GetPoint(1)[2] * scalingFactor);

            glColor3dv(color3);
            glVertex3f(points->GetPoint(2)[0] * scalingFactor, points->GetPoint(2)[1] * scalingFactor, points->GetPoint(2)[2] * scalingFactor);
            
            if (cell->GetNumberOfPoints() == 4) {
                points->GetPoint(3, scalar);
                double color4[3];
                colorTransferFunction->GetColor(scalar[0], color4);

                glColor3dv(color4);
                glVertex3f(points->GetPoint(3)[0] * scalingFactor, points->GetPoint(3)[1] * scalingFactor, points->GetPoint(3)[2] * scalingFactor);

            }

            glEnd();

        }

the code currently displays colors like this

but i want the colors be more transparent so i can see the earth image

[ColeA123]

Yo, id like to email you about your volumetric weather display project. Please reply to me if you can talk about it.

Thanks,
Cole