I don't believe that there is a firm standard that is observed across the industry. I have heard white for voice, gray for "other" voice (like a fax machine or answering machine) blue for data #1 and orange for data #2.
In high-end homes and offices where the feng-sui is an issue, the customer usually wants the plates and jacks to be all the same color, whether that be white, telephone ivory, electrical ivory, dark brown, or some other color. On many occasions, I have had an "open, frank and candid exchange of views" with architects, builders and owners about ensuring that everything blend into the whole picture.
On one such job, I delivered the plates and jacks to the GC, and he took them to a paint shop where they were carefully painted to match all the other plates, outlets and switches, in exactly the color that Madam wanted. On that job, had I used blue for data, there would have been "pulling of hair and rending of sackcloth."
Unless someone specifies a color or colors, I just use white plates and white jacks. I carry a bunch of permanent markers to create black, blue, orange, red, etc, jacks, for special occasions, and I stock a few gray jacks, since that's a color that can't readily be created with a felt marker.
