I have had some of my goldfish change colors just like what you described,
including the lips, fins, sides and anywhere else on their body as they go
from black to gold or whatever other color - if all your tests come up
good, you probably don't have a problem, IMHO
Gale :~)
I have noticed a problem with a Goldfish for a while now. I noticed
that he had black marks on his gill covers and down both sides, just in
certain areas, not all over. This was noticed over a year ago when my
fish returned from my brothers pond, I'd moved house and had to wait to
build my pond. These marks did not get any worse and possibly reduced
slightly until 2 to 3 weeks ago.

The fish started coming to the surface and going straight back down,
bubbles would then come to the surface.

I thought the black marks were possibly signs of a previous problem with
Ammonia although I have never found any trace of Ammonia in the pond
since it was built almost 2 years ago. I thought that the fish coming
to the surface was possibly a sign of gill flukes so used Malachite and
Formalin on two occasions as directed by the labels on the treatments to
try to get rid of anything in the pond that may cause problems.

Today I noticed that the Goldfish has also got black lips and black tips
on his tail and fins. I can't see anything else wrong with any fish in
my pond. I've got Goldfish, Orfe, Tench, Koi and Shubunkin in the pond,
they all seem happy apart from this one fish.

I've checked PH, Ammonia, Nitrate and Nitrite and can't find anything
wrong. Everything is reading 0 except for the PH which is 7.5, it has
been 7.5 since the pond was built, my pond is 17' x 9' x3'.

The pond was well stocked with plants but I've reduced a lot of the
planting to reduce clogging of my filter. The reduction has only taken
place this week and has had no effect on any readings with the test
kits. My filter is rated at 6 times my pond capacity so filtering
shouldn't be a problem. I have not tested the oxygen because my filter
is returning water which oxygenates the pond and I've also got a second
pump running as a fountain, again adding plenty of oxygen. Just to go a
bit further over the top I've got a air pump adding more oxygen.

To cut a long story short and get to the point. Has anybody any idea
what these black marks, lips and fin/tail tips could be?

Thanks in advance.