Congratulations! I would suggest the following for surefire color at your fall wedding: asters (especially valuable if you want blue and white in your wedding), chrysanthemums (keep pinching them back until July to delay bloom), grasses (many ornamental grasses are wonderful bouquet fillers), pansies (for small bouquets and nosegays; pansies love cool weather). There are other fall-blooming perennials as well, but for mass appeal I think I'd vote for those listed above. In all cases, you don't need to start these indoors. Just buy small plants in spring, set them in your garden or in large pots, and by fall you should have plenty of color. What's more, most garden centers carry all these plants in full bloom in fall, so if some of your homegrown plants don't do well, you can purchase replacements.