Any Korean Sweet Potato Experts?

[rejoicing]

I need some serious sweet potato help! 
How do I cook these?
I bought some Korean sweet potatoes from a little Korean market near me.  They are not the normal dry white kind. Some were bright yellow inside.  Those got wrapped in foil and broiled.  Today I went to cut up the others and they were PURPLE inside!!!   When I cut them open, they gave off milky sap, so I want to be sure I cook them right, and don't poison my family!!! 
At the market they don't speak english, and I only know 2 Korean words, so I can't ask them.  Even google fails me!
HELP p&ty!

[orcuttranch]

I've eaten purple potatoes and bright yellow ones too.  We had a home ec teacher in college (long time ago  ) and her husband had his doctorate in potatoes.  He used to travel the world because of it.  Funny.  Anyway - she used to bring in all kinds of weird potatoes for us to cook.  The one that stands out the most was the purple one.  I don't know, though that it was a sweet potato variety?  I found some purple potatoes a few months ago, so we had the kids try them.  I do remember that they seemed a lot more starchy than other potatoes.  Maybe that is what you are seeing?  We just boiled them.  If it makes you feel any better, we're still alive!