I think it depends on the context for whether to include color. If the color is relevant to the content of the page and the image, then I think it makes sense. An example might be something that is unique like "A red lake" but for an ordinary lake saying "A blue lake" may be overdoing it.
I'm also not a viewer of alt tags when I'm browsing, but I would like to hear from someone who is. Do they prefer more descriptive alt tags or more basic? I could see how it might be dull to browse with alt tags being strictly nouns, but if that person has never seen color it may not provide benefit to them.
I agree, context is everything with alt text, and I like your idea of using color when the object is unique.
I'd like to hear from people who use a screen reader as part of their normal browsing process too. There's also the case where an image won't load due to slow connection, and I'm not sure how useful color would be in that scenario either.
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