This demonstrates a pattern suggested here http://bit.ly/h24hXy

testController.groovy

package com.nerderg.test

class TestController {

    def index = {
    }

    def changeColour = {
        withPerson {
            log.debug dump()
            hairColour = 'blond'
            render(view: "changeColour", model: [person: delegate])
        }
    }

    def withPerson(id='id', Closure c) {
        def person = Person.get(params[id])
        if(person) {
            c.delegate = person
            c.resolveStrategy = Closure.DELEGATE_FIRST
            c.call()
            return person
        } else {
            flash.message = "The person was not found."
            redirect action: "index"
        }
    }
}
class Person {

    int height
    int age
    String sex
    String hairColour

    static constraints = {
    }
}