Laravel 5.3表单验证 - 我如何验证选择框？

So I've just started using the Laravel Form builder and validation. I really love it but I can't for the life of me work out how to validate a simple select box. It must be in the docs I know but I can't figure it out.

How do I set the validation so filterColor must always contain either blue, green or red?color

<select name="filterColor">
  <option value="green">green</option>
  <option value="red">red</option>
  <option value="blue">blue</option>
</select>

$this->validate($request, [
    'filterName'        => 'required|max:100',
    'filterColor'       => '??????????',
    'filterValue'       => 'required|max:400',
]);

$this->validate($request, [
    'filterName'        => 'required|max:100',
    'filterColor'       => 'in:blue,green,red',
    'filterValue'       => 'required|max:400',
]);