Note: All pictures are from Forever21 and Victoria's Secret. It's just easier to refer to.
White Fabric/Light Coloured Fabric
You must wear a nude coloured bra for any white piece of clothing. If you wear a white bra, it will accentuate what you are trying to hide in the first place (your bra). For light coloured fabrics, it is best to wear a nude bra or a bra that is of very similar colour to that fabric.
Loose Flowing Sheer Tops
I personally would wear a camisole or tank top underneath these kind of tops. Some girls like to wear just a bra underneath these tops. To each their own though. If you are planning to wear just a bra underneath, I suggest wearing a bralette as they are must prettier to look at than a normal bra. There are different lengths of bralettes. I personally would go for the longer kind.
Muscle Tees/Open Side Tops
I would definitely wear a fun coloured bralette for these. You can wear a bra but I feel like you can have more fun with a bralette underneath peeking through the sides. There is more lace so it's more fun and girly as compared to wear a normal bra with it. Unless it was an all lace bra then go nuts!
Spaghetti Strap Tank Tops
I would wear a bra that would be of the exact colour of the tank top or I would wear a strapless bra. You can also wear a nude bra with this top if you choose. I would try to make sure the bra has very minimal lace as it will show through the tank top if it is form fitting. Bras without lace will be the best option.
Racerback Tank Tops
I would opt for a racerback bra or a strapless bra.
Backless/Low Back Tops/Dresses
Any time you are wearing white bottoms such as shorts, skirts, bottoms and also dresses. Please wear a nude pair of panties. If you wear white, we can see everything! Especially if the material is sheer or slightly see-through in the light. You can wear either a hip hugger or a thong type of panty. It all depends on the cut of the bottoms and how tight the piece of clothing fits on you. The tighter the fabric, the smaller the panty.
That is all I have to say about underwear and how to wear it. If you have any additional questions, please let me know in the comments below. I will be happy to answer.
Also! Please DO NOT show visible bra straps unless it is the same colour as the top AND please DO NOT show any visible panty lines. We all know that it's not sexy or cute at all.