My resident daffodils have changed colour this season from being a deep
yellow
to anaemic yellow.
What could be the cause of this change

belto wrote:
My resident daffodils have changed colour this season from being a
deep yellow
to anaemic yellow.
What could be the cause of this change

Sounds as though they want potash. I'd prescribe a good balanced feed.
After a few seasons, established bulbs will have used up some of the
nutrients in the soil. Similarly, they may be getting overcrowded, which
increases competition for nutrients. I'd try digging them up in July,
when the leaves have started dying down, forking in some _well rotted_
manure or all-purpose fertiliser, and replanting a handsbreadth or so
apart. (Oh, some fresh wood-ash, too, if you've got it: that's good for
potash. Washes away, though: some here may recommend applying separately
in spring for the following year's flowers.)

--
Mike.