@Ajisha To make the engine fill the whole cell rather than a piece of text between opening and closing backColor tags, it is needed to put the closing backColor at the end of the cell, so opening and closing backColor tags to capture the whole cell. For example see the following simple template:
Document doc = new Document("C:\\Temp\\in.docx");
ReportingEngine engine = new ReportingEngine();
engine.BuildReport(doc, new object());
doc.Save("C:\\Temp\\out.docx");