That has nothing to with the bot. That is the color AO uses for the private group. You can change the color of the messages the bot sends to privategroup, but that wont change the color of stuff players say.
I think you can change the color of the privategroup by using a GUI skin. Or by manually setting it. Not sure, but the file cd_image\gui\default\TextColors.xml may be the right one to look at. Not sure, tho. ctch_pgroup looks like it may be the one you want to fiddle with.