Ilford changed their filters some time ago. The colors look different but the filters perform identically. Kodak's filters were transparent in the reds so more visible light got through for dodging and burning. Ilford's filters aren't as sophisticated, but they do let some red through (otherwise the filters would go from green to blue instead of yellow to magenta). It seems Ilford changed the amount of red light the filters let through and so the colors look different.
This color change in Ilford filters seems to be the basis for the idea that the filters are subject to lots of fading with time. I have old and new Ilford sets and recent Kodak sets and sets dating to the 60's - although the visible color may be different the contrast is (near enough to) identical.
Of course filters will fade, ones used in the lamp house of an always-on arc lamp enlarger, such as some monster of a Saltzman, are going to fade a lot. But the intermittent incandescent or fluorescent light from a common enlarger isn't going to cause fading.
Unless you are metering the negative for contrast it really doesn't matter what contrast the filter produces. You take a guess, and if the print is too flat or contrasty you go up or down a grade. What number is on the filter is a real "so-what".
All I can say is my experience is different. I had a set of Ilford filters I used for about 20 years, and the 2.5 did not perform the same as a (never, or slightly) used 2.5 from the same manufacturer. A side by side comparison showed that the old filter had lost about a quarter of a grade. The only enlarger light it ever saw was cold light.
Unless you're trying to reproduce an old image using a print map, the change is probably moot. I likely would never have noticed had that not been the case. Following my notes didn't get me the same contrast as the original print, which is why I used the newer filters. I then did 2 new prints, one with the old 2.5, and one with the new, just to make sure it wasn't an issue with the paper. There was a definite difference.
Originally Posted by Nicholas Lindan
So, Ilford filter go with Ilford paper. What goes with Adox and Arista?
Calumet has good quality, reasonably priced filters. You don't need "super multi-coated" just multi-coated is fine. Nor, do you need pricey German filters, such as B&W. Check their website for availability.