Cameron will only succeed in getting a lot more female and ethnic candidates on the shortlist not actual parliamentary candidates and MPs.
When faced with the shortlist, most of the racist, sexist Tory selection panels will proclaim their liking for more women candidates etc and then will choose white men as their candidates. Their logic will be; it's fine in principle to have more women etc, but in our constituency the best candidate is so and so, who happens to be a white man.
This was the same problem the Labour Party had until it applied all women shortlists. All the best will in the world will not make that much difference until you do something practical that actually works. All women shortlists work, Cameron's approach will look good and improve the Tories image, but it won't make any difference in practise.