Well, as far as I know there's no way to actually set the color by using a SendMessage call. These messages are all just provided by the Messenger client core itself. If they'd need a command which could change the color without the use of a color window, they could add it. But I doubt they would, or even better: I doubt they'd ever need such command.
I am sure there's another, more difficult way to do this but I don't really know how this should be done.
Note: it seems like SendMessage can return a value depending on the message sent. Maybe try to look if it's something useful? If it returns something to control the color window, you could easily set the color you want... I'm just too bored to have a look at it myself at the moment.
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