Getting a bit off topic for a Newcomer's thread really, as the originator's question has been answered.

kbarber wrote:
In fact, the shunt signal replacement programme has made it very common indeed!

(it's been a few years now since I've seen a shunt signal showing red/white for on, but I haven't been a great explorer of the network over that period).

If I remember correctly, the requirement is that you mustn't have shunt signals of the two different types (R/W and R/R) visible to a driver. Thus you have to change all Down signals at a site simultaneously, and all Up signals simultaneously, but you can have Up and Down being different types and you can have different types at opposite ends of a station or otherwise well-spaced apart.

Ah: "Where independent position light signals that display [two reds when ON] are to be added to an existing signalling layout that includes independent position light signals that display a different form of ON aspect, the signalling layout shall be designed so that:
a) Drivers will not observe the different forms of ON aspect at the same time during train movements, and
b) The transition between the different forms of ON aspect is clearly defined."