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Philip,
Providing you end up with an image that is not fully saturated in any part and had an even amount of light (although possibly an uneven flat of course), you can do it at whatever time of sunset/twilight suits your equipment.
You don't want stars or other objects showing through, and you don't want it over saturated so you don't get a correct gradient.
I use a combination of 10 flat frames, averaged. I picked that number because I just generally try to have 10 flat, 10 bias and 10 dark, I figure it's a good number to average out the errors.
My ST7 camera that I do the flats for is only greyscale so I can't comment on the comparison between colour & greyscale. I suspect that there would be little advantage in colour, becuase unless there's something very interesting hapenning optically all the colours would be flat or un-flat in the same way, so a greyscale image would do the job. If there's any advantage in a greyscale image I'm not sure.
Roger.
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