If I remember correctly not all that long ago Ilford Multigrade filters ranged in colour along a fairly continuous progression from yellow (#00) to a fairly dark magenta (#5). This is similar to most VC heads and Ilford still refers to yellow and magenta filtration in the current tech pub discussing contrast control with MG papers.

However the current generation of multigrade filters are not like this. They seem to be a perhaps more sophisticated set of colours. #00 looks orange. There is then the more or less expected progression to magenta through grade 3 1/2. Then a change in colour again at grade 4 onward, which seems (to the eye) to be less "magenta" than grade 3 1/2.

The only reference I can find in Ilford's online resources is a statement that the previous incarnation of the filters was never able to deliver as low a minimum contrast as the current version. Ilford also indicates the dyes may be less stable in the previous version although it is not entirely clear whether that is intended to mean old filters should be replaced, or if it means the newer versions actually have more stable dyes.

Did the current version coincide with the introduction of Multigrade IV? Does it have anything to do with three-emulsion paper vs older versions?