Purple Balls (ondeh ondeh) Delicious Recipes

I was talking to Zue on Sunday about everything under the sun and as usual it will end up with food :) We talked about sweet potatoes and I told her that I bought some special sweet potatoes from Philly Chinatown. Since she too bought some sweet potatoes last week so we decided to make ondeh ondeh together in a different kitchen. Here is my purple version of the ondeh ondeh.

Look like ordinary sweet potatoes
Is purple in colour - steam it until soft
Chopped some gula melaka (palm sugar)
Make into balls
Boil it and coat it with fresh grated coconut
And 'Voila' purple balls
Note : Can someone enlighten me why the boiling water turn into green when I boil the potato balls? There must be some scientific reason for that.