Hi Xpoint! Welcome to the Game Editor forum! I hope you like it here.
The only way to change a text actor's color through script is to adjust the actor's r (= red), g (= green) and b (=blue) variables. Each of those variables can have values between 0 and 255, and every actor in Game Editor has it's own r, g and b values. By changing those values you can change the coloring of any actor, including text actors. But note that if you want to be able to use r, g and b to color your text actor with any color you want, the text actor's initial font should be white, as that's when changing r, g and b can have maximum effect on the color of the text. Same goes for other actors - recoloring works best if the actor's animation is white or drawn in grayscale.
But there is also a different way to change a text actor's color in-game. You can use the Set Text -action in any event, like the mosue button down event, as you suggested. Set Text allows you to change the text actor's text, font and color.
I hope this helps!
And if there's something you don't understand or something else you'd like to ask, please do, I'm willing to help you.