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I have two Punctatus amongst other corys in my community tank. In the last couple of weeks I have noticed their colour is turning a dark brown almost black in colouration which is very differnt from the shiny colour they used to be. Is this any thing I should be concerned about or is it a normal transition.

What color of sands/gravels do you have?

I know sometime, some of the Corys can change their color to make themselves blend in better to their surroundings. They do not change instantly but over the few days, they can either darken or lighten their color.
It happened to me when I had my C.Arcuatus(skunk) in the quarantine tank with black sands(tahitian black moon sands)and they become so dark Greyish color and I can't hardly see their skunk stripe. And they moved to the tank with more light tan colored sands in it, and they lighten their color and become typical color you usually see on their pictures.

However, they also do change their color with other factors such as their health condition, diets .,etc. So probably better if you can post their water parameter, tankmates, feeding and care schedules.,etc.

The gravel is black and has been for some 10 or 12 months so not recent.

They are in a 125 Litre tank with an internal filter and external filter fitted.
PH = 7.0
Ammonia = 0.6 mg/L
Nitrate = 5 mg/L
Temp = 28 degrees C

Tank mates includes: x2 L066, x2 Schwartzi, x3 Otocinclus and x2 Botia Sidthimunki.

I feed frozen blood worm every other night and alternate Hikari food (sinking wafers, algae wafers & carnivore pellets) on the alternate nights. Mornings I use flake. Veggies are used a couple of times a week.

The last change to the tank was 6 weeks ago and was the second L066.

I forgot to add that I change 40 to 45 Litres on a weekly water change.

Ammonia level of 0.6 mg/l isn't quite what you want - that sort of indicates that something has gone wrong with your "friendly bacteria". There should be NO measurable levels of ammonia and nitrite in the tank.

Not saying that this is why the fish as looking "wrong", but it could be.

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Mats

Black substrate is not natural to Corys and they will definitely darken to try and blend in.

Ian