Returns the tint value applied to the color.
If tint is supplied, then it is applied to the RGB value of the color to determine the final color applied.
The tint value is stored as a double from -1.0 .. 1.0, where -1.0 means 100%% darken and 1.0 means 100%% lighten. Also, 0.0 means no change.
property TExcelColor.Tint: Double