what's most important about what a monk wears is the following:

* it can keep a monk warm or cool
* it can keep a monk safe from sun, wind, flies, mosquitoes
* it can keep a monk from showing his shameful parts
* and it can distinguish himself as one who has gone forth into homelessness, as a monk

so the brown/orange is because of the last point, but the buddha also allowed monks to wear rags, even those from cemeteries. from time to time, however, people would offer better clothes, but the buddha would only accept pieces of cloth that weren't sewn or colored like the clothes you and i wear, all for the above 4 points.

No only that Dome.

Brown and orange is the color of the season, Fall.
This shows a changes in environment of the healthy green leaf, turning into brown orange. The color reminds all the monks the meaning of impermanence. Everything changes with time. And by monks looking at the colors, keeps them praising the teaching of lord Buddha.

Orange is the color of fire. Most monks meditate in the forest, therefore, this color is best for protecting monks from wild animals.