Hi Mischa,
Thank you for using Syncfusion products.
We have created the sample at our side and we could see that the particular line color cannot be changed. If we are able to change it using MSExcel then we can do it XlsIO.
We have attched the sample file for your reference.
Could you please update us with the simplified sample so that we can analyze further on this issue.
Please let me know if you have any queries.
Thanks,
Sridhar.S
PlotAreaShrink_918d237e.zip