What is a good ratio to buy bricks in?
Colours

It's difficult to tell which colours are used the most. I have six different colours of bricks that are I'd consider essential for every day city building:


(Monochrome) Black &amp; White
(Primary) Red, Green &amp; Blue
(Secondary) Yellow


I chose these six colours because they are  commonly sold in buckets and tubs and are widely available. They tend to have the best brick to price ratio around and they are also very versatile, you can build most models out of these six colours. They also happen to be the only six colours that can be recognised by a NXT/EV3 colour sensor. 



As your collection builds, you get a better idea about what kind of things you like to build. A builder specialising in medieval models would probably find colours like grey and brown essential. 

Bricks

A good ratio of parts is variable depending on your needs. 

Say for example I wanted to build a the wall in the image below, I'd require 40 1x2s and 8 1x1s. That would be a ratio of five 1x2's for every 1x1. 



Of course, this isn't going to apply to everything you build, you'll need to make an educated guess based on what creation you're trying to make.