You were almost there. Try to do it manually on the command line.
(command ".change" MyEnt
"" "P" "C" "CO" "DIC COLOR GUIDE(R)" 9 "")
The hardest part is finding out the color book name. The named can be found the the xmlk <bookName> field in the .acb file. Manually change the color of an object then query the DXF code, as follows.
Entity data
(-1 . <Entity name: 23555858f60>)
(0 . LINE)
(330 . <Entity name: 235557e91f0>)
(5 . 69DE)
(100 . AcDbEntity)
(67 . 0)
(410 . Model)
(8 . 0)
(62 . 41)
(420 . 16113018) *** True Color (RGB)
(430 . DIC COLOR GUIDE
(R
)$DIC
9) *** Color Book
and color ID
(100 . AcDbLine)
(10 4181.66 3840.88 0.0)
(11 4980.22 3476.59 0.0)
(210 0.0 0.0 1.0)
The Color Book name is a little confusing and requires some trial and error, use 'DIC COLOR GUIDE(R)" and omit the $DIC shortcut name from the end. Or just navigate to the color book location (C:\Program Files\Autodesk\AutoCAD 2018\Support\Color on my workstation), open the .XML color book files and inspect the name.