I know creating copies of a built-in plugin or behavior is bad, I don't suggest you do it at all, but you know this a lot better than me.
Of course, creating a "text_ext" behaviour that would overwrite the draw function could do the trick too, but I have hard time finding a good way to store texts and color in the behavior parameters.
There seem to be another way though, by creating a plugin that would automatically create N text objects and manipulate himself, each text object being of one color only. This would require, for what I know, to link the 2 object types at the start of the layout (a little like this plugin) so the "multicolored text" would know what to create. This could be impossible to propagate the Pin behavior though, no idea, you may know better.
Sorry if this didn't help, I tried my best ^^