I recently bought something that was labelled at the grocery store as a "purple sweet potato". They were larger than what I am used to for sweet potatoes, but looked similar, although they had a purple skin on the outside and white flesh on the inside. I figured since they were labelled as "sweet potatoes", I would prepare them the same way I like to prepare regular sweet potatoes, by boiling them first (for about 40 minutes), then removing the skin, slicing it into wedges and then pan frying them. I have done this many times with normal sweet potatoes, and it works quite well.
However, after I boiled the purple sweet potatoes for 45 minutes, I pulled them out to take the skins off, and they were soft and stringy and incredibly off putting – they felt the same way as a gummy mashed potato would feel, and they fell apart all over the place.
Are they more like a potato, requiring less boiling time? Or is there something else going on here that I am missing?
Edit: I went to the grocery store today, and they are now labelled as "purple yams". I don't know if that makes a difference or not. I also got a picture of them, to make my description a bit clearer. 
Best Answer
I used to cook plenty of purple sweet potatoes when I was living in africa:) Sounds like you maybe cooked them too long...
They can be extremely tasty, sweet with a "perfumy" hint if you are lucky. They are best boiled with the skin on like you did and peeled afterwards (as the taste is preserved better that way). As for cooking time it really depends on the size (which means the potatoes are likely done at different times) but you can always feel when they are done by pressing on the potato with a fork - it should give slightly/be soft. I think it usually took me about 30min but I didn't use a clock. It may shorten the cooking time slightly to make holes in the potatoes with a fork (this also adds to the "creaminess").
I guess they do sometimes get a softer, more "creamy" consistency than your usual potato when well done. However the taste is richer and in my opinion far superior to orange sweet potato. If you prefer a firmer potato, just let your potatoes stay on the firm side (in my experience, though, the taste is enhanced by letting them go soft). But boiled ones would anyways mainly be for puddings as it gets rather sweet.
For panfrying I would suggest avoiding boiling the sweet pots first. Simply peel/wedge them raw, fry them as chips until done and serve with a spicy/hot dip - yum.