Yes, dogs can change colour as their coats 'blow' (hairs. Blowing means that hair strands get old and start to shed, often lighter in shade, e.g. gingerish on a brown dog, but the new coat would be usually darker and shinier. Coats vary in how liable they are to sun-bleaching, and it depends on sun intensity, and perhaps pre-disposingpredisposing factors like being in water a lot.
Coat colour can be affected by foods with pigment in them, e.g. sugar beet, food dyes and seaweed can darken light fur. (myFor example, my palomino horse was ruined for a season's showing by winter sugarbeet feed, (coatas it's coat went reddish), and another Spring after adlib seaweed powder, his hind quartershindquarters grew dark brown fur, which gradually disappeared by end of June, thank Goodness!- I think it dissolved out by rain, as it was not groomed a lot. (He'sHe's normally bright gold in summer, pale gold in winter).
Sugarbeet is often in dog foods (perhaps more as a 'filler or fibre in cheap foods?), and Aa rescue place might likely be using cheaper foods, which may also (I guess!) Havehave food dye added by manufacturers to make the feed look better to human carers (like some dog treats seem to be). The effect of this would wear off inas time passes, according to when it was last fed the color factor. If you knew the exact history of the dog, it may shed light on the feed situation -e e.g. if it was starving somewhere, it may have scavenged dark coloured food-sources
Some species can change colour temporarily for camouflage, and maybe under stress in rescue situations there may be a trigger in some dogs to darken the colour, as that may lower its risk profile, as it might in wild Speciesspecies?
Shelter / rescuerescue people are often wrong (in my long experience as trainer / rescuerescue /dog breed info / behaviourbehaviour work) in guessing breed mixes, and tend to stick to info given by the previous owner, and even first-time owners are given incorrect info, (pups in one litter can have more than one father and not be pedigree as supposed) so the dog may be carrying colour genes /alleles which are not expected. Google dog color genetics for lots of infoinforamtion.Dogs Dogs with sable genes change colour and pattern dramatically while growing up. Merles tend to grow more and more dark hairs, and roan spotting increases with age too.
Good breeders /websites to do with Chesapeakes and Pyreneans may be able to advise on likelihood of colour change through age /feed etc. (GooglePlease google 'Chesapeake retriever coat color' etc.) for more information.
Hope some of this is helpful. Glad he's got a loving home now!