This is probably a problem with fixation or washing.  Either the print
is not adequately fixed (less likely; this would usually give some
silvering instead of fading to a gold color) or was not adequately
washed after it was fixed.

The other possibility is that the print was mounted with materials that
are not acid-free, which will cause fading which is visually similar to
fading from improper washing.

Revise your fixing and washing steps of your process so they are standard
(don't use Ilford's fast-processing recommendations with non-Ilford papers)
and ensure that the new print is archivally mounted.

Gold?  Perhaps more dull yellowish?  Like spots of silver tarnish?  If so,
print is not adequately fixed/washed.  Make sure you are using fresh fixer.
And it certainly does not hurt to tone with highly diluted selenium mix.

This happens to RC paper - the reason for it is a
matter of debate.  Titanium dioxide in the paper
reacting with UV light is one current suspect,
sulfur from pollution or the mount material is
another.

The history of RC materials is one of
problems followed by Kodak/Agfa/Ilford/xxx
claiming 'we have fixed the problem'
followed by either the reemergence of the
old problem or the creation of an entirely
new problem.  I greet any explanation or
solution with suspicion.

Agfa RC was very prone to bronzing.  I haven't seen
any on Ilford RC but that may be a matter of luck/time.

To keep it from happening again you might try using
Ilford paper.  Err on the safe side: use fresh fixer
for 5 minutes, a hypo eliminator, wash for 1/2 hour.
There are all sorts of fast processing methods but
the old methods are known to not contribute to the
problem.

If the picture is in a frame then make sure you
use acid free board for the mat and mount board -
"4-ply 100% cotton museum board" is considered the
safest: the core of the board should be pure white,
not grey.

    This is called silvering out and is common on very old
photographs. It comes from oxidation of the silver image.
The silver oxide is very finely devided and can migrate to
the surface, where it can be again reduced to metallic
silver by other polutants. Very finely devided silver
(colloidal silver) is bright yellow, hense the gold color.
    The problem comes from oxidants in the atmosphere or
from mounting materials. Sources of peroxides are plentiful,
automobile exhaust, vapours from fresh paint, many other
sources.
    Well fixed and washed prints are vulnerable. Actually,
a very small amount of hypo left in the emulsion acts to
protect the image by creating a layer of silver sulfide on
the individual silver grains. However, a much better way of
protecting the image is toning. The best toners for image
protection are sulfiding toners, selenium toners, and gold
toner. While it was found that Kodak Rapid Selenium Toner,
in very high dilutions, was not effective for microfilm a
stronger dilution _is_ effective for pictorial films and
prints where some change in image color or density is
acceptable. According to Dr. Douglas Nishimura, of the Image
Permanence Institute of the Rochester Institute of
Technology, a minimum of three minutes in a solution of 1:9
KRST will protect the image. Another very good toner is
Kodak Brown Toner, a polysulfide toner. Polysulfide tones
all densities evenly so any amount of toning which results
in a visible effect on the image will protect it. Gold
toning is similar but is expensive so its not much used
although its still the standard for microfilm.
    While inadequate fixing and washing can cause image
degradation the effect is most often a brown or yellow
staining. Lack of complete fixing causes overall staining
because the remaining insoluble reaction products are
distributed throughout the emulsion. Inadequate washing can
result in an overall stain but often shows up more as an
imagewise stain. While both stains can be removed the
process can further damage the image and the resusults are
not always permanent. Once an incompletely fixed image is
allowed to age for a few weeks it can no longer be
completely fixed by re-fixing because the reaction products
continue to change in the emulsion, eventually becoming
impossible to remove.
   Despite the problems with inadequate fixing and washing
oxidation of the image is caused by external polution and
well fixed and washed images are particularly vulnerable.
    About 15 years ago some RC papers had problems with
image oxidation caused by peroxides being emmitted by the
titanium dioxide used in the reflective layer. This problem
was solved by adding peroxide scavengers to the reflective
coating and emulsion. The scavengers are self-regenerating
so they remain effective essentially for the life of the
print. These same peroxides also attacked the plastic layer
in which they are suspended causing it to crack and flake.
This has not been a problem for RC for many years now.

   Silvering out can sometimes be removed by a mild silver
bleach, a treatment similar to removing dichroic fog. The
safest method is to treat the print is a bath of fresh rapid
fixer to which is added citric acid in the amount of about
15grams per liter. The print must be watched carefully
because this treatment will also bleach out the image.

    However, the best solution for your problem is to make
new prints and tone them as suggested above. The toning will
protect the prints against even pretty severe oxidative
attack.