Well, keep in mind also that the Empire state building also received a complete LED installation with shifting color patterns, yet no secion of its mast is white.
In terms of using white only to project an image on-screen, this is, as per my limited knowledge of the subject, not quite the case. Most cinemas nowadays have black or darker-tinted screens; and the computer-based technology (i.e., Pixar) to create imagery conducive to more effective "film" projection is (I'd think) because of that.
I mean, why call any good cinematographic production a "film" in this day and age anyhow, when the word "film" refers to the celluloid used for so long? CGI is, of course, today's standard, as is--to your question--LED for exterior lighting.
So as I understand it, while painting the WTC "spantenna" a brighter color would perhaps enhance the LED display, it's not really necessary.
I could be wrong, tho.