The values the color object expects should range from 0 to 1. So if you want to use the more common 0-255 values just divide them by 255 and you'll get the same results.
Also, the following isn't correct, even if you used the correct numbers:
color.green = new color ["RGB", 255, 0, 255];
To replace the old value with a new one, you'd do this:
color.green = ["RGB", 1, 0, 1];
This will change the built-in color green until Acrobat/Reader is restarted. To create a new color as a property of the built-in color object, you'd do something like:
color.new_green = ["RGB", 0, 128/255, 1];
You can also simply set a variable to a color array:
var green1 = ["RGB", 0, 128/255, 0];
event.target.fillColor = green1;
which is what I'd recommend you do.