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it's not necessary to take and apply multiply flats is it?
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Yes it is. Flats contain random noise as well and you want to minimize that as much as possible otherwise you will introduce more noise into your lights.
The best way to do that is to combine multiple images to produce a master flat, the same way you do a master dark. Seeing as your flats would normally only be a few seconds long at most collecting multiples is not a problem. I use a manual bulb cable and just lock it open for a while and collect a couple of dozen of them and then select the ones with the closest histogram spread.
As for the colour, if you really want to go to the extremes you can split your lights into RGB components, split your darks into RGB components, split your flats into RGB components and then process them separately then recombine them, but only if you really like to punish yourself with processing.
Using them as greyscale should be fine especially if your software debayers your lights initially anyway (ImagesPlus does this under the automated control, not sure about others).
Just remember if you take flats more than a few seconds long you will should really take flat darks as well.