public bool IsCustomColor {get; set;}

//var voxelLayer = ... ;
//Must be on the QueuedTask.Run()

var variable = voxelLayer.SelectedVariableProfile;

//Change the color of the first surface for the given profile
var surface = variable.GetIsosurfaces().FirstOrDefault();
if (surface != null)
{
    if (voxelLayer.Visualization != VoxelVisualization.Surface)
        voxelLayer.SetVisualization(VoxelVisualization.Surface);

    //Change the iso surface voxel value
    surface.Value = surface.Value * 0.9;

    //get a random color
    var count = new Random().Next(0, 100);
    var colors = ColorFactory.Instance.GenerateColorsFromColorRamp(
        ((CIMVoxelStretchRenderer)variable.Renderer).ColorRamp, count);

    var idx = new Random().Next(0, count - 1);
    surface.Color = colors[idx];
    //set the custom color flag true to lock the color
    //locking the color prevents it from being changed if the
    //renderer color range or color theme is updated
    surface.IsCustomColor = true;

    //update the surface
    variable.UpdateIsosurface(surface);
}

//var voxelLayer = ... ;
//Must be on the QueuedTask.Run()
//var variable = ...;
//var surface = ...;

if (surface.IsCustomColor)
{
    surface.Color = variable.GetIsosurfaceColor((double)surface.Value);
    surface.IsCustomColor = false;
    //update the surface
    variable.UpdateIsosurface(surface);
}