Many years ago, I realized that once I wore something to a funeral, I never wore it again. It wasn't a conscious thought, but I simply never wore those clothes again. Now I own a gray funeral dress and it only comes out for funerals.
I'm so glad other people feel this way.
Before last year, I had only been to one funeral - my grandmother. I had just turned 8 when she died, and I wore the dress she had made me a little while before she died - a beautiful crimson velvet party dress with white lacy cuffs and a net petticoat. It had been made for Christmas, but she passed away at the beginning of December. I never wore it again after the funeral.
Last year, I went to 5 funerals. My best friend's father, my godfather, my brother, my uncle and a close family friend. The day before my best friend's father's funeral, I bought a black dress with a pattern of white butterflies near the hem. I then wore that for the other 4 funerals I had to go to that year.
The idea of wearing that dress to any other occasion makes me feel physically sick, so it's just hanging in my wardrobe, waiting. It's a pity, because it's a beautiful dress and suits me perfectly, but the association with my grief is just far too strong to be ignored.
As for general funeral dress codes - to be honest, I don't really notice what other people are wearing at a funeral, unless it's really over the top. So long as they're present and behave respectfully, I don't think it matters what they wear.