how to get the field names for a table

["Honglei Tu" <honglei@...>]

Hello,
Thank you for the answer of previous question.
LHF.
Another one:
If I have a Lua table which is
color["red" = 1 "green" =5 "blue" =6].
I wonder how to get the field names and put them
in a set. I want to get ["red" "green" "blue"].
I will write a Lua function to get them.
But is there any way to do that ?

Thank you
Honglei