Paint chips for white balance cards?

[KmH]

I know you believe you understand what you thought I said, but I'm not sure you realize that what I said is not what I meant - Robert McCloskey.

 

[runnah]

As mentioned our brains are easily fooled. Bright colors look darker against dark colors and vice versa. The light in which you view colors make a difference as well.

Better off shelling out some cash for a proper card.

 

[Gavjenks]

I'm not of the opinion that you really need or should use a gray card at all.
It helps you get your color balance right in-camera, but if you shoot RAW, you can pretty much always fix it fine in post-processing, unless you make a tremendously far off error in-camera in coloration.
It also helps you match the scene more "realistically," by having a reference point for what was actually there. But when you really get down to it, why does that matter? Are you doing crime scene forensics? If so, strict reality is important, go get your gray card. If not, I think it's fine to just color the scene later to what LOOKS realistic, and often you will actually be able to make the colors look more realistic than reality actually did in post-processing yourself, based on what humans EXPECT to see in that situation.

Like, maybe you're in a room at sunset, but you're taking an interior shot, and it's not obviously sunset from image cues. In such a situation, I might actually want my final image to be much bluer than reality, because otherwise the room has a weird cast from the sunset, but the viewer has no way to know it's supposed to be sunset, so it ends up just looking like you did a bad job photographing the place. In such a situation, the way it "should have looked" reads as more pleasing and accurate than the reality that the gray card would lead you to.

So I just shoot RAW to give myself latitude later, whenever I can, and then color it in post processing to be whatever I think that scene should look like, without worrying a bit about what it was.
(NOTE: if you shoot a whole series of images like for a client in a photoshoot, choose one color adjustment for the whole set and apply it evenly! You can choose what looks real to you, but only once. Otherwise it will look tacky and inconsistent)

 

[gsgary]

If its for when using your studio light set you camera to the temperature of the lights

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[Gavjenks]

Hence my note at the bottom. Most RAW converters allow you to set one temperature correction and then just auto apply it to further photos until you tell it otherwise. Which has the same unifying effect on a studio shoot as gray carding beforehand would.
The only difference should be that the gray guard guarantees a reference point for "realism" whereas eyeballing it in post once and then applying consistently is based on your own subjective opinion of what looks most realistic. And usually I think the latter provides better results anyway.