Conditionalformattings can apply other colors than just .Color. You can also apply theme, index or auto color.

Here's an example of applying theme color using .Theme instead on a AboveAverageRule:

using (var pck = new ExcelPackage())
{
   var sheet = pck.Workbook.Worksheets.Add("NewWorksheet");

   var cf = sheet.ConditionalFormatting.AddAboveAverage("A1:B5");

   cf.Style.Fill.BackgroundColor.Theme = eThemeSchemeColor.Accent4;
}

The same logic applies to most other conditional formattings. With the exception of Databars and Colorscales.

As of Epplus 7.0 databars and color scales have built in support for theme, index or auto color options.

Here are some examples of applying theme colors on

A databar:

var cfDatabar = sheet.ConditionalFormatting.AddDatabar("A1:B5", Color.Red);

cfDatabar.FillColor.Theme = eThemeSchemeColor.Accent1;

A ThreeColorScale:

var threeColor = sheet.ConditionalFormatting.AddThreeColorScale("A1:B5");

threeColor.LowValue.ColorSettings.Theme = eThemeSchemeColor.Background1;
threeColor.MiddleValue.ColorSettings.Theme = eThemeSchemeColor.Accent2;
threeColor.HighValue.ColorSettings.Theme = eThemeSchemeColor.Accent3;

Note that there are some differences in how to set the colors due to ensuring backwards compatability. And because Databar has multiple options on one whole whereas colorScale has different needs as these two are more complex and diverse types.