Diverging data colours between Power BI Desktop and PowerBI online

This is not by design. Customizations in Power BI Desktop should be preserved in the service.

Also, just a note, 'Diverging Colors' is an option if you are using a visual with color saturation, and might confuse people trying to help you. I understand that your problem is that the color definitions you apply to the legend entries in PBI Desktop are not preserved when uploading the .pbix to PBI Service. Your terminology may confuse someone not reading too closely.

Make sure you submit this as a bug report, as well. You can do so in PBI Desktop by using the 'Send a frown' option in the File menu, or by opening a ticket here for bugs in the PBI Service.

The forums are a great avenue for discussing issues, sharing thoughts / ideas, and troubleshooting, but they are not the official medium to submit bug reports.

Proper bug reports will receive more attention from the devs than forum posts.