This snippet of code should work to write a message to the chat in a specified color:
Code:
function WriteTextColor(color, text)
Turbine.Chat.WriteLine(string.format("<rgb=%s>%s</rgb>", color, text))
end
-- Usage:
WriteTextColor("#FFFFFF", "Hello, world!")
-- Make sure to provide the first argument as a hex string representing the color
EDIT: Whoops, didn't notice that you said 'channel' - I was thinking chat in general then. :P