Controller not running

Hello again

Ive been attempting to create my own controllers for custom doctypes but none of the code appears to be running.

 from __future__ import unicode_literals`

 import frappe

 from frappe.model.document import Document

 from frappe import _

 class Colour(Document):

         def validate(self):

             frappe.throw("controller running")

im not sure what ive done wrong here