In Lotro LUA plugins, the first number is the alpha value, then RGB follows. The numbers range from 0 to 1.
For fully opaque colors, always start with a 1.
Grey would be Turbine.UI.Color(1,0.9,0.9,0.9). To make it darker, change the 0.9 values to something lower, making sure to keep all 3 values the same.
For gold, I got a gold HEX code from this site: http://www.htmlhelp.com/cgi-bin/color.cgi
The value for gold was FFD700 in hex.
Quick answer is this translates to Turbine.UI.Color(1,1,0.84,0).
To get these numbers, translate each part from hex to decimal, then divide by 255.
Edit: Found this site
that sets the background color according to the values entered, can be useful for finding the correct numbers you need.