Like I said, the way most imageboard software (including 4chan) does it is to use the appropriate color for OPs and replies respectively

It does mean the "wrong" colors appear if a user isn't using the default color scheme, but that's 1. exactly how 4chan and other imageboard software does it, and 2. much less of a problem than the current situation where thumbnail backgrounds are always black and anything dark is invisible

Now generating multiple sets of thumbnails does sound stupid - half the reason behind using JPEG thumbnails in the first place is to reduce the amount of storage space they take up, and now the server would be having to work several times harder every time an image is uploaded!