[IcED]

I think its 'Font Colour', the ones in every program code.

[MeterFirefoxDuble]
Meter=STRING
X=2
Y=12
FontFace=Palatino Linotype
FontSize=11
FontColor=255, 255, 255, 200
StringAlign=LEFT
StringStyle=Bold
AntiAlias=1
Text="Firefox"
Hidden=1
MouseOverAction=!Execute [!RainmeterShowMeter MeterFirefox]
MouseLeaveAction=!Execute [!RainmeterHideMeter MeterFirefox] [!RainmeterRedraw]

Just a thing, you need to use the colour's number and NOT its name.
You'll need to find out black's colour number.