Hi Nick,
Sorry, I'm a little late to the party on this one (busy weekend!)
Due to the way they were produced, we know that both Palitoy and Kenner figures were usually produced alongside one another (no real surprise there). Therefore, without wishing to disrespect the frankly superb Variant Villain website, which quite rightly points out that Obi-Wan's hair colour isn't simply white or grey, but has a multitude of shades in between, to you or I, and I suspect most collectors, we know what we generally regard as white or grey when we see it. And on that basis, I recommend using the also superb Mr Palitoy's guide (
https://mrpalitoy.orgfree.com/), which confirms what I already knew to be the case from my own childhood memories and subsequent purchases, that the white haired version came first, and it wasn't until the Palitoy 41B that we first got a grey haired one, after which both white and grey were available on almost all cards. His website does of course focus on Palitoy cards and hence doesn't cover POTF, but it's a pretty fair bet both colours were probably also available on that too
Jeremy