My adventures in color continue...
I have found that the ColorFormat object changed in O2010 when a Brightness property was added. It seems this is used by Excel instead of TintAndShade. I had not noticed this before because I am, primarily, a Word developer. Whilst the Brightness property is there in the Word object model I have never had occasion to use it as the color of most objects can be set using a combination of ObjectThemeColor and TintAndShade.
Excel is different - TintAndShade works on some objects but not all. Where it doesn't work the Brightness property is used instead and takes the exact same value as TintandShade.
So to set my series markers to a theme color my code needs to select the series and run the following code twice:
With Selection.Format.Line
.Visible = msoTrue
With .ForeColor
.ObjectThemeColor = msoThemeColorAccent3
.Brightness = 0.25
End With
.Transparency = 0
End With
However, this still leaves the fill colour of the marker which stubbornly refuses to accept that it is theme aware once changed in code (i.e. change the theme from the UI and the marker fill colour doesn't update).