Let's say model wears dress number 1, I setup my lights to light setup 1 and we hold a gray card a shoot it and set the white balance of camera on that. Now if model changes to dress number 2 and I change my light setup to setup number 2, do we adjust the white balance again? Which one has more effect on white balance? Dress or light setup or both? In light setup scenario for example imagine: 1- SAME modifiers, just moving them lower, higher, left right.. 2- Different modifiers, so for example from a white reflective umbrella to a beauty dish and grid
The white balance is determined from whatever light is reflected from the grey card.
Does altering the color of a dress change the color of what is reflected from the grey card? No. Does the grey card still look grey? Yes. The dress (that's behind the grey card or not even in the picture) doesn't change the white balance that you determine with the grey card.
Does altering the light setup change the color of what is reflected from the grey card? Possibly. This depends on how you change the setup. If you bounce your light source off a wall for example, the color of the light changes depending on the color of the wall, which in turn changes what is reflected from the grey card thus the white balance has to be adjusted again.
white balance = reflection from the grey card
It's important to know that whatever influences the reflected light from the grey card will influence the white balance. If you let the model hold the grey card next to the model's dress so that light reflected from the dress hits the grey card, this will change the light that is reflected from the grey card and thus change white balance. You usually don't want that. This is why you should keep an eye on what light hits the grey card. If in doubt, only photograph the grey card without the model (or colorful dresses) to be sure that none of that influences your white balance.
The light and the dress both have influence on the white balance, because light reflects off of the dress. In practice, most of the light usually comes directly from the light source, walls, or reflectors you have. However, sometimes you will need to compensate for the color of the subject as well.
Scenario #1: You are taking pictures of a model in an ordinary studio. The model switches from a black dress to a white dress. You keep settings the same.
Scenario #2: You have been shooting pictures for an hour. The color from your lights is slightly different now. You need to adjust your white balance. (For some lighting, especially less-expensive lighting, the color will drift during a shoot. This applies especially to continuous lighting, but strobes also drift.)
Scenario #3: You are shooting in a room with white walls. You move to a room with yellow walls, and bring your lights with you. Even though your light is the same color, so much stray light is bouncing off the yellow walls you need to adjust the white balance.
Scenario #4: You are shooting close-ups of a model. The model changes from a black dress to a bright red dress. Enough red light is reflecting off the dress onto her face that it makes her face look unnaturally red. You adjust the white balance to compensate, or adjust the color of her face in post.
There are no solid "rules" for when or how you should adjust your white balance. A gray card isn't a replacement for trained eyes and artistic sense. Do use your eyes, and if it's important, look at shots on a calibrated monitor.
A very good question. The WB settings are nice to get them right from the start. And you will develop an eye for the display preview if you like the setting you chose or not. More important from my point of view is to take the shots in RAW format so that you can adjust perfectly in post processing. For the rest I agree with the other posts: If you don't change the light source and you have set the WB (not automatic) you don't need to change anything. Good luck with whatever you're shooting next!