donut chart: data.color function broken

https://c3js.org/samples/data_color.html

the above example demonstrates real-time modification of chart color via `data.color = function () {}`.
if you, in that example, change the chart type to `donut`, you'll see the color modification is broken -- it does not respect the generated color from the callback.

it does not respect the generated color but uses the legend color instead.