Hiya I'm Sikh. So will give you the basics
Ideally you should be covered head to toe. Hence why Indian outfits always have bottoms with it. You may feel uncomfortable if everyone else is covered and you're wearing a dress. But if you do. Please try to wear one that is below the knee and has a bit of give. There is a lot of bending over and bowing. So for modesty sake...😂 A longer dress or trousers are easier to sit on the floor with also.
You will need a headscarf. Mainly to cover your head. You can go sleeveless if you wish. Many people especially the younger generation do so.
Take a small amount of money- doesn't matter how much. As when you go into the hall, you would be expected to bow and place a voluntary contribution to the temple. Most people just give small amounts.
You would also be expected to sit on sides according to sex. So will be apart from your husband.
Take a small paper fan as the halls times to get crowded and overheat quickly...especially as we have a tendency to invite everyone and the milk man to weddings.
If you wish you could change for the reception. I only do so if I'm a close family/friend. At the reception anything is fine. You wouldn't be expected to cover your head.
If you want there are online market places that sell Indian clothes. Diya online. Has some decent stuff. If you want to buy from a shop go anywhere where there is a large Asian population. But if you don't see yourself being invited to another Asian function anytime soon, I wouldn't bother.
Alternatively if you have any Asian friends you could always borrow.
In terms of colours. They say to avoid black/white. But I think if you have a mix of colours it's fine. I.e. white blouse and yellow skirt. Also as most brides wear red, I would avoid this. But again mixing I feel should be fine.