If you look under the PostLayer Object in the Help, all the commands you can do are listed in the Properties and Methods tables. Click on Symbol and you'll see that Symbol returns a MarkerFormat object. That means that all commands for the MarkerFormat object also apply to the Symbol.

Click on the MarkerFormat object and you will see the options for the color and index (shape within a symbol set) and all other options.

For example:

Set MapFrame = Plot.Shapes.AddPostMap(SurferApp.Path+"\samples\demogrid.dat", LabCol:=3)
Dim PostLayer As Object
Set PostLayer = MapFrame.Overlays(1)

PostLayer.Symbol.Set = "GSI Default Symbols"
PostLayer.Symbol.Index= 6
PostLayer.Symbol.FillColorRGBA.Color = srfColorRed
PostLayer.Symbol.LineColorRGBA.Color = srfColorCyan

PostLayer.SetSymbolScaling(Method:=srfPostLinear, SymbolHeight1:=0.1, SymbolHeight2:=0.5, ScaleValue1:=1, ScaleValue2:=100, HeightCol:=3)

PostLayer.SymAngle = 315

PostLayer.SymFrequency = 2

See also the attached script PostMap.bas, which can also be found in the \Samples\Scripts\ folder in the Surfer installation directory.

Updated November 1, 2018