Ну, если вы используете взаимодействие ...
var app = new ApplicationClass();
app.Visible = Microsoft.Office.Core.MsoTriState.msoTrue;
var myPresentation = app.Presentations.Open("c:\\test.pptx",
Microsoft.Office.Core.MsoTriState.msoFalse,
Microsoft.Office.Core.MsoTriState.msoFalse,
Microsoft.Office.Core.MsoTriState.msoTrue);
var slide1 = myPresentation.Slides[1];
var range = slide1.Shapes[1].TextFrame.TextRange;
range.Font.Color.RGB = -654262273;
И не забудьте
System.Runtime.InteropServices.Marshal.ReleaseComObject(<your com objects here>)