In the Settings.xml file under Simatic-Hmi-Common > Services >CustomColors_TrueColor is a Base64 (RFC1421) coded binary entry. The format of the decoded binary in decimal format is 1,255, Red_1, Green_1, Blue_1, 255, Red_2, Green_2, Blue_2,..., 255, Red_16, Green_16, Blue_16, where Red, Green, and Blue are the 0-255 RGB values of the custom colors. For example, the binary code:Af8Ao+D/W2Zw//////////////////////////////////////////////////////////////////////////8=is for two custom RGB color values 0, 163, 224 and 91, 102, 112. I was able to create a script to take a table of my custom RGB values and encode them into Base64 binary that I can then paste into the settings.xml file. I can copy the XML SettingNode "Simatic-Hmi-Common" from TIA V13 Setings.xml into each of the other software version Settings.xml files to have the same custom colors in all versions. I can also add this XML node to other workstation Settings.xml files. 

Simatic-Hmi-Common XML node: