This is a pretty obscure area of salmon fishing so if you aren't a salmon angler it isn't something you might normally know.
Salmon start to lose condition once they enter fresh water and so fresh fish are most valued and considered best eating. As they spend more time in fresh water not only do they lose condition but they also turn a darker colour, fresh run salmon are often described as being like a "bar of silver." Now, photos are not always a great indication of this as certain light and angles can sometimes make fish seem a lot more coloured than they are, or even less coloured, and I've seen photos of fresh fish which made them look like they had been in for a long time.
The image below is of a very coloured fish. Once fish start to get coloured it is normal to return them as their quality for eating will have degraded so with moderately coloured fish it is a sort of judgement call as to whether to keep the fish or not. The one in the photo above actually looked less coloured in real life than in the photo and it was very good eating so the flesh hadn't degraded to any great extent and it probably hadn't been in fresh water for as long as you might guess from the photo.
A very coloured salmon which has clearly been in fresh water for some considerable period and which would not make great eating, it was returned immediately after the photo was taken:
