ok, basic rules for formal weddings - no white, cream, black or red. (black meaning it looks like you aren't happy for the couple, white/cream are trying to compete with the bride, red was traditionally the colour of the groom's mistress) Also never look like you are trying to look like one of the wedding party unless you are part of the wedding party.
Is this a wedding when you will need to change for the evening reception or is it a whole day event? It's not normal to change for the evening anymore.
What's your budget? Basically, you need to look nice, formal but not like you are trying to pull or are a bridesmaid.