What was different about it that makes you think it wasn't haddoni?
There are some pics of different carpet species in the Anemone FAQ at the top of this forum.
This is a good carpet picture thread:
http://reefcentral.com/forums/showthread.php?s=&threadid=201498
You can see pictures of my two S. gigantea, for reference, in this thread:
http://reefcentral.com/forums/showthread.php?s=&threadid=201498
There are some markers to look for if you can't tell what species it is. Presence of different coloured verrucae (bumps on the column), for example - haddoni don't have contrasting verrucae; but only haddoni have variegated patterns. The tentacle size and shape can also help.
To be honest a red carpet of any species, if healthy, is worth jumping on. FWIW I've never seen a red gigantea and if there is such a thing, that would be a VERY impressive find. Red haddoni's, to give you an example, retail for $400 and up south of the border.
Purple, OTOH, I have seen in both haddoni's and gigantea's. But purple is also a somewhat more rare find as well.
Can you go back and take a picture of this one? I'd love to see it.