Many camera have inbuilt manual white balancing to use white / grey cards to get the base colour from. Beyond that Take a picture of the white thing, leave all data in RAW format, and then in Photoshop or whatnot you can just select the white card with a white balance selector tool, check the numbers, and batch apply them to all images.
The greycards come in various forms. Normally pure black, 18% grey, and pure white. If you take a photo of them under the same lighting as the subject you will have a full set of details for your exposure and colour compensation. The camera can take the exposure of the grey card, and often the colour balance too. If not the colour balance can automatically be selected based on these cards.
Basically if any part of your scene should be white, medium grey, or black, then with most graphics programs you can set it to use that point as a colour reference to remove colourful casts. The actual grey cards only serve to provide a reliable reference.
hey if worst comes to worst use a peace of white computer paper. this should solve the problem. makes sure to white balance it though. I would recommend a C-sand and a white card.