IN version 6 you can change the colour of the IE text box quite easily. On about line 84 of skin_file.inc you'll find:
Const strIETextBoxColour = "#FFFFFF"
Change the colour in there and away you go.
Changing the colour of the text area for non IE browsers, and the subject line in general is more complex, and requires changing atleast the relevant pages that display the text boxes, and maybe the style sheet as well.
Check out
my forum running v6. I've also created a mod on that that allows registered users to select different skins - all with the correct background colours in the IE text box.
hth