Syntax highlighting was introduced in version 6.5 and has been enhanced in 6.6 (theme and editing)
Syntax component Color Example
Commands Blue ...timechart
Command arguments Green ...timechart usenull=false
Functions Pink ...timechart count
Keyword modifiers and Boolean operators Orange ...timechart count BY productName
I agree. In the code behind the scenes (which you get into if you write your own custom spl commands) keywords are key value pairs (this=that) and options are non-key value pairs (I liken them to switches or arguments in command line).
So in this example
| rex mode=sed field=source "s/replacethis/withthis/g"
The keywords are mode=sed & field=source and the option is the sed command.