Firmatta is no use to you.
Simply send three bytes from Processing and pick them up with a serial read and then transfer them to your WS2812 buffer and get the library to show them.
I know that they could get out of sync but I have found this not to happen.
Look at Serial Input Basics - updated - Introductory Tutorials - Arduino Forum
Note the colour on an LED will not look exactly the same as on the screen. This is due, amongst other things, to the screen being gamma corrected and your LEDs not being.