I think you don't see them because those are "angry" colors. I can see this particularly in HH since they seem to be a complete system, an enclosed system. I know they do studies on this for tents and find people respond worse in situations where they're regulated to that area for long periods of time than with say blues or greens or tans, yellow actually isn't as bad as red but people seemed to be more sooothed by the blues and greens. So if you were stuck in a storm inside your hammocktent you'd be more likely to become irritable with those angry colors and the calm colors... allegedly.
So that's my hypothosis but I'm sure it's just one of many reasons.
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