dear post office: are dark brown envelopes okay?

According to this, light type on a dark envelope can cause issues with the OCR reading the envelope, which may lead to delays or misrouting:

http://pe.usps.com/businessmail101/addressing/deliveryaddress.htm

For my sister's bachelorette party, I had black envelopes with silver writing - no problems and no additional postage

Edited to add: I was a Rural Letter Carrier for 5 years and saw some pretty amazing things make it through the mail system.

All of mine mailed just fine

I think as long as the writing is legible, it shouldn't be a problem.

Honestly, as long as the zip code is clear it will make it to the destination. But yes, as long as it is legible, it will make it. I have ordered items where my address was written in PENCIL on a black box and it made it on time.

You seriously can mail anything as long as they can read the writing. My dad was a carrier for over 30 years. His complaint is when people write in cursive. Just make sure it's legible, especially the name and zip.