Thanks for your interest in Aspose.Slides.
I like to share that color of legends in chart are set on the basis of the chart series color set. So, whatever is the series color the same will be set for legend. I regret to share that at present the support for automatic color for chart series is unavailable in Aspose.Slides and one need to explicitly set the chart series color. An issue with ID SLIDESNET-34799 has been created in our issue tracking system as new feature request to provide the support for automatic chart series color setting. Please visit this documentation link for possible options at the moment. Please share, if I may help you further in this regard.