@Andrey_Potapov Thank you for pointing me to the ColorTransform options.
Tint & Shade are actually not the options that match the behavior of PowerPoint.
Fortunately, there are other color transform options that match PowerPoint’s behavior.
For example, for an Accent 1, Lighter 80%, use the following transform options:
shape.FillFormat.SolidFillColor.SchemeColor = SchemeColor.Accent1;
shape.FillFormat.SolidFillColor.ColorTransform.Add(ColorTransformOperation.MultiplyLuminance, 0.2f);
shape.FillFormat.SolidFillColor.ColorTransform.Add(ColorTransformOperation.AddLuminance, 0.8f);
Similarly, for Accent 1, Darker 25, use the following:
shape.FillFormat.SolidFillColor.SchemeColor = SchemeColor.Accent1;
shape.FillFormat.SolidFillColor.ColorTransform.Add(ColorTransformOperation.MultiplyLuminance, 0.75f);