Open-sourcing is IMHO very unlikely. 1) Pocomail uses commercial modules that couldn't be used in open source project, ie. third party modules. 2) Pocomail is developed in Delphi. I have watched some Delphi programs being open sourced in the past and it didn't work, IMHO Delphi and open source doesn't go together (one thing is Delphi is not free software either).
As for the color coding, this is a quick background, hopefully correct: color coding aka marks is stored in the index file, thus it will not be easily exported. Pocoscript it seems can't read marking, can only assing marking. I have checked filters and filters can search for "marked as".
So one possible way to handle this would be to find out how your new program implements marking. Eg. it might store it as a custom message header, that would be easiest. In such case you could then run manual filters on each folder where if certain mark is found for a message, it would add appropriate custom header to that message that the target email program could recognize. That's just very raw way / general idea on how to possibly approach this.
I'm also assuming that Thunderbird is lightweight compared to Pocomail, and that Outlook may in fact be the only choice for someone who uses email the way I do.
That all depends on your needs etc.. Thunderbird is quite able email client, it also has many add-ons etc. Maybe also check EmClient and Postbox (based on Thunderbird).