dye preview before apply?

i was reading through new updates, and said i can actually preview how it will look like before i apply.
i have yet to find out how to do this thing.
does anyone find out how?
do i can do this before i buy from npc?
do i have to own dyes to do this preview thing?

thanks

You need to own dyes to preview .

Just double click your dye and click on the item you want to dye. Rather than applying the dye, it will pop up the preview display and give you the option to cancel, or apply the dye(s).

ah~ thx guys
but this thing isn't really preview if i have to own dyes lol