Custom Liveries in Gran Turismo are like a rewind button.  It can be used for small good but for the most part it will be abused as the rewind button will be abused.
Allowing someone to paint a Ferrari Hot pink is like allowing some slackers to "express" themselves by spray painting the Eiffel Tower.
It's called upholding the integrity of the simulation.  Once you allow a livery editor that gives you the ability to do whatever you want to a Ferrari and other road cars you've not only cheapened the simulation you've just made all the real world paint schemes available for each car obsolete and worthless.
Cars coming in a limited amount of colors is,  again,  realistic and a proper simulation.  If you add a livery editor that can be used on road cars you might as well not give factory colors as an option when buying a car.  You might as well have a color wheel that allows you to choose any color under the sun which of course threatens the integrity of the game and simulation aspect.  For example,  a Ferrrari F40 should NEVER be any color other then red.  When racing online I don't want to race a lime green Ferrari F40.  The moment that happens I won't be able to take GT5 seriously anymore just as I dont take Forza seriously
If,  for some odd reason Kaz gives in to the casuals and includes a livery editor I hope there is some option to make all the cars online look like their original selves. 
 I dont want to see poke-cars or halo posters on cadilacs.  They can keep that to themselves.