I just read an article on ars technica
about colour blind gamers, and it occurred to me that I haven't seen any thought about that issue here, I can't say I have the answers but it is something we should think about.
Off the top of my head possible problems are distinguishing between growth (green) and production (red) for people who have severely red/green colour blind. Throwing in a category icon would help, but I'm not sure if this one actually is an issue, its pretty clear that orbital factories is production, and nanotech medicine is growth.
Distinguishing between stars, no idea how bad this is without being colour-blind myself, differing sizes helps but the red and the brown(orange?) stars may be an issue (the pics on the article suggested that some people see red as brown). Possibly if we have a way of identifying the worst offenders* we could replace them with obviously distinct replacements in a "colour blind mode", say turn brown stars into symbolic 4 point star, or a different colour. Even if it looks bad it still doesn't interfear with gameplay.
Empire colours, probably the biggest issue we have. Not only do you have different empire colours but two empires can have different shades of the same colour. Compounding the problem is just how important an empire's colour is, on the galaxy window empire colours are used exclusively to tell what ships & stars belong to who. This defiantly could be improved: Possibilities include giving each empire a flag/choice of flags with a distinct visual motif, Ships icons could be a silised sprite based on the actual design of that races' ships. However those are probably only effective if you zoom in. Another option is an override empire colours, the player chooses a personalised set of visually distinct colours, no matter what colours the players choose for their empires, on the colour blind player's screen they come from the list.
* Anyone know of a way to make X.org simulate colour blindness?
Then there are other issues, partial sighted and the fully blind (if there even is a way we can make freeorion playable with a pure text/screenreader interface) Or deaf players. That too is something we should be thinking about.