I'd like to just clarify that out of the two solutions, solution b is my preferred one. The ability to recolour a line using /aline means that extra functionality is also added: You can recolour lines by line data rather than line number (equivalent to using $fline() in conjunction with /cline, but much less awkward to write out).
It also makes sense in this situation, because the intent of a /aline -n command is to have a line added if it is not already added. And under the circumstances that you attempt to add a line that already exists, your intention is probably to also 'refresh' that line.. that would be a refreshing of the colour.
That's my little rant, though this is kind of a feature suggestion and bug report in one because I am quite certain the original intention was that the line colour not be changed. Of course as I just dsecribed, there are some advantages to allowing the colour to change..and since its already half implemented...
