Indeed it is payware, although (there's always one

)
You can download the demo version and use it for an unlimited time, the only thing you can't do is save any changes you've made in the program itself.
However, you
CAN use it to preview the paint.
Just reload the aircraft in FS Repaint (F4 default) everytime you save a change to the paint.
Confused?
It's 6:42am and I havent slept...I am!

Cheers,
Carl