Validators are registered with the validators extension. The validators extension accepts an array of validators where the key is the name of the validator and the value a callback function that receives the value as parameter and returns a boolean.

Kirby::plugin('your/plugin', [
  'validators' => [
    'isGreen' => function ($value) {
      return v::in($value, ['green','lightgreen', 'darkgreen', 'mediumgreen'];

In your templates, you can now check if the value is in the given array of green values:

$input = 'lightgreen';
if(v::isGreen($input)) {
  echo 'Yay, valid!';
} else {
  echo 'Oh no, not valid';