I think the examples you refer to were to illustrate how 'marron' doesn't change in gender or number as other colour adjectives.
As you will know by now, there are exceptions to all rules in French, personally, I have never heard of brun being used for dogs or mice...
Hope this helps!
Maybe this helps: https://www.quora.com/What-is-the-French-for-brown-Brun-ou-marron
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