Pieter's spot on with IDing your current images and their locations in collections.
Another solution if avoiding smart collections - have six collections, with say No.6 being the one you put all images into initially Then delete from No. 6 when you place in 1-5.
I think the label and keyword ideas will work, but they require the extra work of assigning the color or KW and then, if I remove a photo from its portfolio, removing the color or KW.
That's the wrong way round to remove things. Use smart collections and remove keyword/label/whatever and it will then vanish from collection.
Adding a keyword and/or colour etc to a file to make it appear in a smart file is possibly the easiest way to get files into organised collections and it doesn't have to be complex. Have the rule: keyword contains 'P1' [or P2 etc] and use that to add images to your smart collection. Maybe have an equivalent of the sixth collection where all images are collected by the rule: keyword contains 'Portfolio' then in that collection of all your portfolio work add the 'P1' or 'P2 etc. That way you can rearrange what goes in what section from one location.