monotight – 6.35mm (¼") Female x 6.35mm (¼") Female Push In Joiner

(These are grey now)

This quality made fittings are made from POM / Acetal engineering plastic which had high chemical and heat resistance. 

It's also able to hold relatively high pressures in excess of 150psi.

 These fittings are common and compatible with many home water filters, reverse osmosis systems, water filters, fridges with water dispensing etc.