The expiration date on dye powders and solutions is merely a guide and
is no better in some cases than the date on processed milk. If you rely
entirely on the expiration date then you may on the one hand step in zoo
doo, or on the other hand throw away a valuable and still useable
product.
As far as I can see the only rationale for disposing of dyes past their
expiration date:
1.    If they do not consistently work on a standard control tissue.
2.    If part of the certification for the procedure is that it is
mandatory to use dyes that are within this specified date.

It is true that some dye powders and solutions deteriorate over time.
This may be due to a variety of factors such as oxidation, moisture, UV,
contaminants and so on. Some powders and solutions improve over time,
after all the discovery of polychrome methylene blue was because a
solution was inadvertently left on a shelf exposed to air for several
months.
Because of problems with deterioration I have often wondered why dye
powders cannot be sold as small amounts in a dark sealed glass vial or
in a nitrogen atmosphere.
Glutaraldehyde and osmium tetroxide stock solutions for EM can be
purchased in sealed vials. We constantly throw away hematoxylin
solutions that are out of date but this is chiefly due to storage. If a
ripened Hx solution is placed in a bottle with a layer of mineral oil on
the surface it can be kept for several years without deterioration.
At a time when we are polluting our own environment it is something we
should all think about.
Barry