George Lucas intended the ships in the prequels to look better than those in the original trilogy but to be less technologically advanced than those of the prequels. The idea was to show how Palpatine was making the galaxy more militaristic and utilitarian. The ships are all white or gray with a little black thrown in versus say the sleek design of the Naboo Starfighter with the silver and yellow or the colorful Jedi Starfighters. It also shows in the way they dress too, compare the outfits Leia wears to those of her mother.
I thought that had more to do with the difference in ANH’s and the prequels’ budgets. 
pragmatist!
“Although the difference between plot and story may seem arcane, it’s quite crucial: The plot is merely a sequence of events, whereas a story limns those events’ deeper motivation and meaning. The plot gets characters from point A to point B; the story makes us care.”
FTW! Thanks for the link, Armando.
That probably played a part in the difference but that’s pretty much the way George described it in the behind the scenes stuff on the Phantom Menace DVD.