Both green and grey are for telecoms and communications however, grey ducting is specially approved for use on the national BT network, whereas green ducting is used for all other types of telecommunications such as Virgin media cable TV and fibre optic systems.
Grey ducting, commonly known as BT ducting, as this is its sole purpose, is a solid wall uPVC duct and conforms to BT's specifications. It is resistant to most corrosion and common liquids and chemicals. It is always smooth walled and does not come in a twinwall variety.
Green ducting which is externally corrugated with a smooth internal bore (known as twinwall ducting) made from HDPE is mainly used for CCTV and other fibre optic communications. It is manufactured in accordance with dimensions and performance requirements, tried and testes by the telecommunications industry.
The smooth wall green PVC-U ducting is often used for cable TV and is approved by virgin Media.You cannot use green ducting for any BT telecommunications applications!