Indian weddings/celebrations are usually pretty glamorous - women pull out their best silks, fancy jewelry (and loads of it), lots of bright colors etc. For Indian parties, we throw out the whole "just wear 1 statement piece of jewelry" rule and will wear a huge blingy necklace with huge blingy earrings, arm loads of sparkly bracelets, rings etc!
Like Maya said, white would be a no no. Black can sometimes look dull in a sea of gorgeous jewel tones but it would work if it was glam - maybe sequins? I wore a black sari to a wedding a couple of years ago but it was heavily embellished and of course I piled on the jewelry too so a world away from a simple black sheath dress!
Personally, I have always found it odd to see westerners wearing a sari or traditional Indian garb - it just seems so costumey sometimes. Sorry, I don't want to offend - just share my opinion.
If you're not keen on buying something, I would just wear the fanciest dress you own and like Ruchi said, focus on some glam accessories - maybe some sparkly chandelier earrings, bling embellished sandals etc.