Although dress norms vary from town to town, boys typically wear suits and girls skirts/dresses to the service and, if the party doesn't follow immediately, whatever degree of formality in dress is appropriate to the celebration.
There are no colors that are off-limits in the synagogue for religious reasons - it's more important to be appropriate, meaning (for girls) nothing too short, too tight, too revealing or provocative, etc. If the party invitation did not specify "black tie", but it's also not a swim party, you're in that mid-range where the kids will wear anything from "nice casual" to their synagogue clothes. Have your daughter ask around to find out what other kids are wearing.
Also, when at the service, she should not put a prayer shawl - they're for adult Jews only, who say a special prayer when putting them on. She could put on a yarlmulke, but it's not required for women and most don't. MitzvahChic is the #1 bar/bat mitzvah planning book and website! Visit http://www.MitzvahChic.com and be sure to sign up for FREE planning reminder emails.
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