Chapter 55 of the Rule of St Benedict says:
"Let clothing be given to the brethren according to the nature of the place in which they dwell and its climate;
for in cold regions more will be needed,and in warm regions less. This is to be taken into consideration, therefore, by the Abbot. We believe, however, that in ordinary places the following dress is sufficient for each monk:
a tunic,a cowl (thick and woolly for winter, thin or worn for summer), a scapular for work,stockings and shoes to cover the feet.
The monks should not complain about the colour or the coarseness of any of these things, but be content with what can be found in the district where they live and can be purchased cheaply.
The Abbot shall see to the size of the garments, that they be not too short for those who wear them, but of the proper fit.
Let those who receive new clothes always give back the old ones at once, to be put away in the wardrobe for the poor.
For it is sufficient if a monk has two tunics and two cowls, to allow for night wear and for the washing of these garments;
more than that is superfluity and should be taken away. Let them return their stockings also and anything else that is old
when they receive new ones.
Those who are sent on a journey shall receive drawers from the wardrobe, which they shall wash and restore on their return.
And let their cowls and tunics be somewhat better than what they usually wear. These they shall receive from the wardrobe
when they set out on a journey, and restore when they return."
This was written in the 7th century and at that time monks would be seen wearing a mixture of colours - some habits would be grey, some brown and some natural wool. By the 12th century, however, the various monastic orders had settled on specific colours which continued throughout the middle ages: Benedictines and Cluniacs wore black, Cistercians and Carthusians wore natural off-white (the Carthusians wore a hair-shirt next to the skin); absolutely no monks wore brown from this point onwards. Brown and grey habits were again introduced from the early 13th century - but for friars rather than monks.
Have a look at this thread for more information:viewtopic.php?f=10&t=20246