opening new instance of cygwin from withing cygwin

I'm using cygwin on win7, What I would like to do is something like this:

cygstart cygwin tail -f /foo/test.log | perl -pe 's/error/\e[1;31;43m$&\e[0m/g'

I know I can start a new instance using either of these:

mintty -e ...
cygstart tail ...

But neither of those open in ANSI mode, so I can't do color-coding for keywords.