They said that during the race, the silver will fade away due to the rotation speed of the wheel and it will look like a plain black tyre, so you can make a good difference between medium (white) and hard (wich will look black). Pirelli also stated that some tests were made to make sure that the colors this year can easily be distinguished by everyone.
"...and there, very much in flames, is Jacques Laffite's Ligier. That's obviously a turbo blaze, and of course, Laffite will be able to see that conflagration in his mirrors... he is coolly parking the car somewhere safe." Murray Walker, San Marino 1985