: Haven't used it in a kayak,

I have - its very hard to work, and quite a lot stiffer than pine or WRC.
I probably wouldn't use it again for a large area of the deck as I did on
my Hybrid Cormorant:

: but if you use it for the color, beware -- it doesn't stay purple!
: It will change over time to a dark, purplish brown.

The photo above shows the deck on almost its first launch - it had changed
to a dark purplish brown as soon as epoxied.

: The UV protection we put on our kayaks may slow it down, but maybe not.

The boat has now has fifteen sea days, and probably thirty or forty days
out in the sun, and as there are a few details still niggling on the boat,
it still hasn't been varnished. The colour doesn't seem to have changed
very much yet on the purpleheart - maybe a bit darker, though the pine
has darkened a great deal and the WRC somewhat, so its hard to tell - the
contrast is still holding up well. It still looks purple in certain lights.

One thing to watch is that it is not very absorbent, so epoxy doesn't soak
in well. I wiped it very thoroughly with white spirit and then meths before
glassing, and it seems to have adhered well ...

Andy