It isn't always possible to match variables with regexes in syntax defs (which Sublime uses to create scopes, which in turn color schemes use to highlight stuff). That's the case for python; you cannot reliably tell variables apart from other tokens.
However, it works for Powershell, for instance:https://bitbucket.org/guillermooo/power ... age#cl-266
You normally use "default" syntax defs, not customised ones, yeah, that was a manner of speaking.
More info on syntax defs:http://sublimetext.info