main.css

.in-service {
  color: #00FF00;
}

.en-route {
  color: red;
}

.to-hospital {
  color: blue;
}

.on-scene {
  color: gray;
}

.out-of-service {
  color: orange;
}

units_helper.rb

module UnitsHelper
 def set_status(status)
   # Converts the status (if it's not nil) to a parameter version
   # "In Service" => "in-service"
   status.try(:unit_status).try(:parameterize)
 end
end

unitsviewindex.html.rb

<td>
  <div class="<%= set_status(unit.status) %>"> <!-- sets the div class to the parameter version, like <div class="in-service"> -->
    <%= unit.status.try(:unit_status) %> <!-- outputs the text of the unit status, like "In Service" -->
  </div>
</td>