White balance settings can keep the colors of your photos accurate, but they can't make up for a truly accurate color reading. You can manually set the camera's white balance using a simple white piece of paper, but depending on how you expose the shot, it might not get quite right. That's when you need to turn to the go-to accessory of all color-savvy photographers, the 18% gray card.
A gray card is a neutral balance tool. Instead of basing color adjustments on white or black objects, neutral balance determines the best adjustments using the average of all the light in an "average" scene. The 18% gray card will let the camera and any post-production software know what color levels the overall exposure should produce. White balance using a white object can produce mixed results, because any overexposed image will appear white.
Place the gray card where the subject will be, so the light hitting the gray card is the same as the light hitting the subject. If shooting a portrait, have the subject hold the gray card in front of their face for a test shot. When you process your photos later in Photoshop, look at the test shot. Enter the Image > Adjustments > Levels menu, and click on the middle eyedrop icon. Save the level adjustment, and then load it in every other photo under those lighting conditions. You've just color corrected your shots with a gray card.