1. Let the light source illuminate the gray card. Doesn't have to be next to it - just lit by it. Overhead lights in an ice rink light up everything in the rink with virtually the same color cast, so you could put it on the floor at your feet and shoot it even.
2. Shoot the gray card. Just zoom in on it, take the shot using just the rink lighting, and you're good to go. It doesn't even have to be in focus.
3. Set the WB to use that shot as the source. If you don't know how to do this, see your manual. It's a couple clicks at best on the menu. Once you know where they are and how to do it, it's a total breeze. It will then use that WB until you tell it to do something different.
4. Done. Shoot to your heart's content.
The gray card is more reliable than random white sources for best accuracy, because random white sources can have their own color cast which we can't readily detect with out naked eyes, but can affect overall WB to the camera's more sensitive WB detection system. A photographic gray card has no such color cast issues.
Can you explain step 3 more? Im lost. Are you trying to differentiate white and grey?
Nope. Not trying to differentiate white and gray. Trying to get a correct color balance to the photos I'm about to shoot.
I'll try to explain...
All light has color, measured by temperature. When it's blueish, we say it's cool. When it's reddish, we say it's warm. Incandescent lights produce a yellowish light. Flourescent lights produce a greenish light. We call that color cast - the color that a particular light makes a photo look. You've probably seen that in plenty of photos - sometimes they look to yellow or too green or too blue or whatever.
Cameras are 'tuned' to a particular gray used in gray cards. It used to be 18% gray, but now I hear it's 14% gray or something - doesn't matter, as far as I know. When you point a camera at a gray card and tell it (with a few buttons pushed) "Camera - you're looking at a photographic gray card" then it knows what color that's supposed to be, and it can tune to it.
So, imagine you have it looking at a gray card, but you're shining a red light on the gray card. You tell the camera with a couple buttons on the menu, "this is a gray card - adjust". The camera will adjust it's color temperature so that the photo that comes out will still look like the proper gray card that it knows it's supposed to be. It won't be red.
So, no matter what subtle color is in the lights of the room you're in, the camera will make the adjustment based on what it sees with the gray card and the light hitting it, and you'll get a photo of a gray card, the way it's supposed to be - tuned.
After the camera is tuned like that, everything shot in that light will be the right color in the resulting photos. Skin is the right color of skin, per person, blues are the right shade of blue, and all the rest. The photos don't come out too yellow or too green or too blue or too red or too magenta, etc. They come out just right.
That's how the camera tunes in with the light it's shooting in - through the reflection of light on the gray card.
Now, if you want to know which buttons to push or what menu settings to access to do that, you'll have to check the manual for your particular camera. Just look up "setting custom white balance" and follow the steps. It's real easy.