My grandpa was a Presby minister, and he said "no black!" for his funeral, too. Bright colors, suitable for celebrating a life (and, if you believe in it, an afterlife), were best.

You could always wear a colorful hat. Bottom line, though, is that anyone who dares to criticize you for what you wear is a jerk. It's a memorial service, and it's totally fitting that you would want people to share happy memories about how much they enjoyed having your mom be a part of their lives. And also to dress the part.