Very thin white line in colortbl

Yes this is due to the coloured panels being snapped to pixel boundaries.

You can go \\\noalign{\vskip-0.1pt} for a suitably small amount between each row, then the panels will overlap and you are less likely to get a gap. You can add set \extrarowheight to something large to compensate (which is probably a good idea anyway, to keep the capital letters away from the top of the panels)

That is for horizontal lines. For vertical lines \rowcolor has optional arguments to specify optional left and right overhang.