i know i am late for this answer but the correct answer seems to be
See :help getcompletion():
:echo getcompletion('', 'color')
which you can assign to a variable:
:let foo = getcompletion('', 'color')
or use in an expression register:
:put=getcompletion('', 'color')
This is not my answer, this solution is provided by u/romainl in this post on reddit.