This attribute defines the display color of the segment's ephemeris.
Astrogator <Satellite Object Path> SetValue <Attribute Path>.SegmentColor <Value>
The color is defined in hexadecimal format, as 0x00BBGGRR, where BB is blue, GG is green, and RR is red. Connect does not acknowledge hexadecimal numbers, so once you have chosen the hexadecimal expression of the desired color, you will have to convert it to a decimal number.
To display the segment in red:
Astrogator */Satellite/Satellite1 SetValue MainSequence.SegmentList.Launch.SegmentColor 255
STK Programming Interface 11.0.1