Conditional Formatting ‐ Colors

$$\color{#ff4545}\textsf{\Largeⓘ\kern{0.2cm}\normalsize Below only available from Epplus 7.0 onwards}$$

As of Epplus 7.0 several of the ConditionalFormattings have built in support for themeColors

for example here's how to theme color 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;
}

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