The purple refers to the type of sweet potato (purple skin, white flesh), not to the eventual colour of the soup (un?fortunately).

Peel the sweet potatoes and chop them up into small pieces. Cover them with water, add a teaspoon or two of tom yum paste, and boil until the sweet potatoes are mushy (use a potato masher at the end if necessary). Adjust to the right consistency with extra water, add extra tom yum paste to taste if needed. Just before serving, stir through chopped fresh coriander.

I know this sounds weird but it's very yummy! (and easy).