Ah, well, it depends if you're talking about the in-game "PA Forums", or the
http://pirate.planetarion.com "PA Forums".
I've been playing with the in-game forums, and i've not come across your problem yet. Though the in-game forums have recently changed, making playing with it entertaining, if you could provide some more detail as to what skin(s) you are using in-game, and any other information you think is relevent, i might be able to be better able to replicate your problem and work on a solution.
As for the "pirate" PA forums, editing your post should not be a problem. Generally, i've found that the skins i've been playing with all show the edited text as black on a white background, which are - when posted - then changed to white text/black background or whatever the skin you choose to use applies to it. Eg, the Lairyfish skin that i love so dearly types in black, but displays in white on a deep green colour once its been posted.
If what you are doing is trying to type in white colour, then when you choose to edit it, it might be hard to see. To be frank, dont use the coloured text options when posting; let people's skins play with the colours. Eg, some skins (like Pro Vizion) have white backgrounds, which makes posts which are in white text hard to see; but in default (automatic) colour would display in black.
This is essentially the jist of the reply, and the result of my own experiments. I apologise for not replying earlier.
Hope that helps anyway, please post below with clarification if it does not.
<3