I think I probably knew about red filters being good for contrast in skies, but I still didn't really know what yellow filters did for you or when you might use them.

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So I got some Rollei bayonet filters which conveniently fit my Yashica 635 and I went out and shot each one side by side on the same scene to see what they results would be.

The answer - red good for landscapes, but NOT for portraits. Yellow, good for pretty much anything. Green filter - no idea. Can't see a great purpose for this one over yellow and red. But I'm all ears if anyone has any great suggestions for when a green filter can be useful!