The only other thing you may have to watch out for is making sure that you're not having the red channel being undercorrected as the screens "white" is blue biased.
Probably won't be an issue as long as the flats are well exposed
You can simply take a shot at clear sky before sunset. Tracing paper can additionally even out possible small clouds...
Flats will be converted to and treated as BW in DSS.
colour etc doesnt matter for flats, but yeah a sheet of tracing paper to diffuse would be good. One problem you might get this way is artificial vignetting thats not present if you took flats unfocused off the sky.
besides flats, darks and bias frames are NOT required, and for most people will make little difference to their final image. so dont fuss too much.