The color function takes an array of colours. For the scatter plot this equates to one colour per data group.
if you change it to
nv.addGraph(function() {
chart = nv.models.scatterChart()
.showDistX(true)
.showDistY(true)
.color( [d3.rgb("green"), d3.rgb("orange")] );
};
it seems to work as you need.
EDIT - setting colour using RGB string
this can be done using css syntax
nv.addGraph(function() {
chart = nv.models.scatterChart()
.showDistX(true)
.showDistY(true)
.color( ["rgb(0,255,0)","rgb(255,165,0)"] );
};
You would do well to read the API documentation at https://github.com/novus/nvd3/wiki/API-Documentation