I think on cloudy days, contrast is the key. All of the colors tend to turn dark, so you need very good contrast. Everyone's eyes are different and can see certain colors better. Personally I do not see yellow very well in low light conditinos while others do. I personally like a darker color contrasted with white. Others prefer darker contrasted with yellow. Figure out what colors show best for you and the develop a high contrast scheme with large color areas. Lots of small lines do not show up well and they all just blend together,

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