The command to change box color is:

/wincolor <BOX> <red> <green> <blue> <transparency>

BOX = whichever box you want to change... CHAT, PARTY, BUFF, INV, etc. (yes, they have to be in all caps)
red = value for red, 0-255
green = value for green, 0-255
blue = value for blue, 0-255
transparency = value for transparency, 0-4 where 0=completely transparent and 4=solid

The one I use is /wincolor CHAT 0 0 25 3.. a nice dark blue color, with a little transparency.

White would be /wincolor <BOX> 255 255 255 4
Black would be /wincolor <BOX> 0 0 0 4

Hotkeys for Windows:

ALT – B: Buff/Spell Effects Window
ALT – P: Party (Group) Window
ALT – C: Chat Window
ALT – S: Spell Gem Window
ALT – H: Hotbox Window
ALT – T: Target Window
ALT – Y: Self-Status Window
ALT – M: Menu Window
ALT – L: Lock down window position