How to select a subgrid row in an expandable ui grid?

Question

I'm implementing an expandable ui grid. When a parent row is expanded, I want a row in the subgrid to be automatically selected based on the parent row's data.

How do I get a list of subgrid rows? Is there a way to find and select them with gridApi.selection methods? I can set isSelected = true if I have the row, and I can get a list of rows (that doesn't include any subgrid rows) with $scope.gridApi.grid.rows, but I don't know how to get a list of subgrid rows.

$scope.fruits = [...];
$scope.colors = [...];
$scope.gridOptions = {
  data: $scope.fruits,
  enableRowSelection: true,
  expandableRowTemplate: '<div ui-grid="row.entity.subGridOptions"></div>',
  ...
  onRegisterApi: function (gridApi) {
    $scope.gridApi = gridApi;
    gridApi.expandable.on.rowExpandedStateChanged($scope, function (row) {
      if (row.isExpanded) {
        row.entity.subGridOptions = {
          data: $scope.colors,
          ...
        };
        if (row.entity.color_id) {
          // need help here
          _.forEach(/*rows in subgrid*/, function(colorRow) {  
            if (colorRow.entity.id === row.entity.color_id) {
              colorRow.isSelected = true;
            }
          });
        }
      }
    });
  }
};