LOL what?! your boyfriend stole it?! That really is crazy... hehe sorry!
Ok onto your question!! Light Flush and Petticoat are very different- I tried posting a comparison in the swatch thread earlier on this year, but it doesn't show that well on camera...
Light Flush is a LOT lighter than Petticoat!! Petticoat is definately blush material- it's pigmented enough to show up on its own (excessively) depending on your skin tone. While Light Flush can also be used as a blush (on me), it may turn out better on those who are darker than NW15 over a blush, rather than by itself. That being said, Light Flush can be used as a highlight I believe- while there's no way that Petticoat can be, since that dark raspberry veining is DARK!!
In comparison, I think the two MSFs are notably different enough to justify getting both if you can. They're nowhere near dupes of each other, and are quite unique! The texture of Light Flush is very soft and finely milled- while I find petticoat to also be very soft, it does have some glitter in the gold veining.
I know that Light Flush was on the mac website till recently, so if you're anywhere near a CCO (though what would I know... I'm in Australia of all places lol!) I would suggest you take a look there! I'd expect you to be able to find one there, seeing as how it never really 'sold out' as such online, and it's off the website now!
Hope that helps!
ETA: I did actually swatch Petticoat and Light Flush next to each other in the Sheer Minerals swatch thread- the difference isn't that pronounced in the photo- but as you can see, Light Flush is sort of... peachy/shrimp pink as compared to the darker more raspberry like tones of Petticoat.
