I don't use a steamer, just a pan of boiling water.
There's a difference between non-toxic and edible. I use chalk on things that aren't going to be eaten. To me, a gumpaste flower is not edible, especially not once it has a wire inside, so I use chalk on that. I also use markers on gumpaste, the non-toxic crayola markers. I don't use chalk on anything that will be eaten and if I use the markers on something that may be eaten, I warn them to remove it from the cake first.
Here's what the markers look like on gumpaste (the details on the baby and blanket):
The flower petals were dusted with wilton's pearl dusts.
In this picture, the peach rose and the red rose were dusted with chalk:
The pink rose was dusted with wilton's pink dust.
These roses were dusted in the centers only with ivory chalk for a really subtle effect:
You can paint gumpaste too, when you need deep colors: