Hello Uwe,

Allow me to first say that this is unexpected behavior, and I have logged this in our internal tracking systems so a developer can take a look. The pink color that you are seeing appears to likely be used for internal testing purposes, but accidentally made it through to the UI of the chart. I have logged this issue with a development ID of 255632 and have created you a private support case with an ID of CAS-197073-B9H5K9 that I will link to this issue, which you can access here.

As a workaround to this behavior, I would recommend that you handle the TooltipContentUpdating event of the series that your CategoryTooltipLayer is meant to represent. This will result in an event handler that expects a return type of Control, and you can use the following sample code to prevent this behavior:

private Control Series_TooltipContentUpdating(object sender, ChartTooltipContentEventArgs e)
{
   DefaultTooltipControl control = e.CurrentContent as DefaultTooltipControl;
   if (control.ParentForm != null)
   {
      control.ParentForm.BackColor = Color.Black;               
   }
   return e.CurrentContent;            
}

I am attaching a sample project to demonstrate the above. I hope this helps.

Please let me know if you have any other questions or concerns on this matter.

UltraDataChartCategoryTooltip.zip