The ripening itself is caused by ethylene gas released by the food, as you mentioned, but really this can be trapped by any kind of bag. There is nothing inherently special about a brown paper bag itself, but it's porous nature allow small amounts of ventilation, whereas plastic ziplock bags do not have this property and thus do not work in the same way.
The temperature of the fruit effects the speed at which it ripens, so if it is at higher than room temperature it may ripen faster but there is an increased chance of rotting.
The colour of the fruit really depends on the type of avocado as a "green skin" avocado does not vary much as it ripens whereas a "Hass" will.
I think this question comes down to more than just a yes or no answer. You have to take into consideration a larger number of factors including the type, time of year (i.e. season), etc. But in most cases avocados will ripen faster in paper bags.