If the color never changed and the pattern never came out then the original owner has bad water quality or something is stressing it out.
My reason being, in the second photo you posted you can clearly see it's patterning, so there is marking there.
Here is a recent Congi I picked up. (my Congi count is 7 so far)
Note this pic is her in the bag from a white tank environment:
Here is same fish a week or so later after settling in my well established tank: (it's the one on the left)
bro thx for the advise.....bro but the colour very nice golden leh....if stress then the colour like this nice,then i always stress him better...hope u dont mind,doesnt mean anything,just say what i think...
Anyway don't have to stress him to achieve this colour, got healthier way like WTT.
Personally I don't like this type, more markings the better. If want this kind, find authentic xanthic nicer.