The Official Highway Code: reflective road studs -Rule 132

The Highway Code applies to England, Scotland and Wales and is essential reading for everyone.

Reflective road studs may be used with white lines.

White studs mark the lanes or the middle of the road.
Red studs mark the left edge of the road.
Amber studs mark the central reservation of a dual carriageway or motorway.
Green studs mark the edge of the main carriageway at lay-bys and slip roads.
Green/yellow studs indicate temporary adjustments to lane layouts, e.g. where road works are taking place.

http://content.govdelivery.com/accounts/UKDSA/bulletins/9ec360?reqfrom=share