Doing 3 small colour studies taught me that colour can quickly capture the mood. Capturing colours as they appear at the time rather than as I think they “should” be is what makes a day scene different from an evening scene. If I was merely replicating the colours I expected to see, this would be lost and I would be left with the same image.
Subtle differences are harder to capture than for example colours at sunset vs colours at day time but it’s certainly not that only the sky is purple – the whole image appears to have a “filter” of purple, all colours are of purple tint.
Paradoxically, colours are in a sense “truest” or most as you’d expect them on an overcast day. Probably this isn’t paradoxical at all, since there is no harsh light of any kind to affect the appearance of objects, hence we’re left with the flat colour of the object itself, reflecting off of it. So perhaps although an overcast day isn’t great for shadow studies or a strong painting full of contrasting colour pairs, it’s good for colours in their “most real” form, least affected y the lighting conditions.