The green refers to the Green Ribbon Club,the HQ of the Whigs
before,during & after the Succession crisis (1681-85)
and the Glorious Revolution (1688-89),
who defended parliamentarianism against the attempt
of King James II to impose an absolutist régime.

The Whig leaders (Essex, Russell & Sidney)preferred
to chose death rather than deny the principles of freedom.

The blue was (and is) the colour of the Tories,
who sided with the monarchy and its measures
to install a despotism, suppressing free speech,
abolishing local government & coersing religious conscience.