I've also wondered, as the platys we have were labelled as marigolds in the shop (which sells a huge variety of platys, so I kind of assumed they should know... perhaps that's a bit naive...!) - but I've also seen pictures of very similar fish labelled as sunsets.
According to 'What Fish?' by Interpret publishing, 'The marigold platy has a basic colour of yellow, deepening to orange towards the rear, becoming blood red at the base of the tail'. This, and the accompanying photo, exactly matches our marigold platys. I wish they'd stay still long enough for me to take a photo, but all I have is pics of the aquarium with a blurred orange splodge across the middle...
Our marigolds have developed deeper colours since we got them, especially the male. They started out small, and fairly uniform orange, a little darker orange towards the tail, but now they've grown, their faces are orange, then the body is yellow, then darkening back to orange about halfway along their bodies, darkening to red at the base of the tail.
Pictures of sunset platys seem to be similar to the marigold, but more of a yellow, even silvery colour on the body, and with the face the same colour as the body rather than orange. Just to confuse us, there also seems to be a 'neon' variation of either, with a blue-silvery sheen overlaid on the body, and also your Mickey Mouse variation, with the black markings overlaid on the basic colour.
There are some pictures of the two sorts on the following website (sunset is near the bottom of the page):
<a href="http/goliadfarms.com/pages/products/fish/livebear/platies/main.htm" target="_blank">http/goliadfarms.com/pages/products/fish...laties/main.htm</a>
If anyone knows better, though, I'd be very interested to find out the correct definition!