The serious don'ts are obvious. Unless there was a specific request from the deceased or the family, refrain from
- too casual wear like blue jeans, sneakers or graphic tees
- Bright cheery colors or prints
- Trendy outfits
- Short hemlines. Keep things modest, this is in no way about you, dress as such!
Just remember the ceremony is not only about celebrating the deceased's life but also to bring confort to the family. Try to keep your outfit tame, modest and your attitude to match. You should be discreet and conforting to the family/friends.
Sometimes people try to sound conforting and end up saying the wrong thing. Phrases like "he's in a better place now" can often hurt more than confort. One way to never go wrong is to express how sorry you feel for the loss and offer to do anything you can. Most people won't take you up on it, but it's just the right thing to do: hug, express sorrow, offer help.
Somtimes people will feel unconfortable thinking they need to wera head to toe black but you don't. If you want to, go ahead but if you don't just make sure to keep it somber and simple. If there is a certain level of formality, try to heed it and not be the person who showed up in jeans when everyone else is in dresses.
I've got a couple suggestions for different degrees of formality. For the top formal, try the somberest simplest dress you own, likely a solid black one. Unless it's too short, you can make it more modest with a cardigan and plain dark shoes.
If it is too short do yourself a favor and wear pants. Dark pants, shoes and a simple top (blouses are a good choice) and if modesty requires, a cardigan, are ideal. This fits all kinds of formality, unless you've been told about a very specific dress code. If you have, heed it, out of respect.
Otherwise if it's a more casual thing you're looking for like for mourning outside of ceremonies, you can even go with dark jeans, as long as everything else is "right", you'll be fine. Dark patterned blouses, solid tops in navy, chocolate or dark gray can all be worn without reserve.
If you have any aversion to black, try another somber neutral like the ones specified before. In a pinch or during a particularly hot summer, even light gray can work. I've even seen light blue and white work!
The main rules are the plainest accessories possible (or consider not wearing any), keep shoulders and knees covered and everything else about you extra discreet. This includes shoes, hair and makeup. And don't forget your nails! This kind of thing usually catches you off guard but you don't want to show up anywhere with neon nails!
Black is not mandatory, respectful is.
What about you? Do you have any advice for other people in the same situation?