Depends on how soft.
If they are green throw them out (mind you, how you could tell with purple potatoes, I'm not sure).
According to a report in the Daily Mirror (British tabloid newspaper - aimed, apparently, at a consumer with reading age of 10!) purple potatoes will stop you getting cancer
Baked purple spuds might help people beat cancer.
The superfood from South America now available here is packed with antioxidants which kill off tumours.
In tests on mice the potatoes, usually the size of a golf ball, halted the growth of bowel cancers by targeting their “master” stem cells.
Prof Jairam Vanamala, of Pennsylvania State University in the US said a starch in the potatoes boosted the immune function in the gut.
And this could cause cancer cells to “self-destruct.” (I thought purple was an appropriate colour to paste the cutting
)