First, create AlertDialog from builder:
AlertDialog dialog = builder.create();
Then you can find your button and change color:
dialog.show(); //Only after .show() was called
dialog.getButton(dialog.BUTTON_NEGATIVE).setTextColor(your_color);
dialog.getButton(dialog.BUTTON_POSITIVE).setTextColor(your_color);
dialog.getButton(dialog.BUTTON_NEUTRAL).setTextColor(your_color);
Here you use next method:
void setTextColor (int color)
Sets the text color for all the states (normal, selected, focused) to be this color.
Parameters
color int: A color value in the form 0xAARRGGBB. Do not pass a resource ID. To get a color value from a resource ID, call getColor.