In computer RGB systems, Neutral is defined as having equal RGB components.

White is RGB(255, 255, 255)
Black is RGB(0, 0, 0)
Middle gray is considered to be RGB(127,127,127) if in linear space, however gamma makes it be around RGB(186,186,186) in histograms. But equal if it was neutral gray.
Other lighter or darker gray neutral shades have varying but equal RGB components.

See Wkipedia: RGB Color Model

A color cast would make one or more RGB components have a different number, not equal. A little or a lot, but not equal. There does exist gray tones (paint for example) with pink or blue casts, NOT neutral. Neutral is special, equal RGB, and no color cast.

So the whole idea in White Balance is that if we photograph a KNOWN neutral white or gray card, and the lighting or incorrect WB setting gives it a color cast, the WB tool clicks it.
Clicking it tells the computer "We know this spot is neutral, so make it be neutral". And it does, and it is. Result is equal RGB, i.e., no color cast. This makes correcting bad WB results be mostly trivial to do. More about that at http://www.scantips.com/lights/whitebalance.html

Clicking an orange card would also make the orange spot be a neutral gray, but of course, that totally spoils the picture. With orange removed, picture would be decidedly blue overall. But if it were a neutral white card turned orange by wrong lighting, this click would instead fix it to be perfect.

Orange is about the typical color of incandescent lights. That light gives the neutral card an orange cast (if seen in say Daylight WB setting). Clicking WB on it removes the orange, resulting picture is corrected so that neutral shows as neutral. An accurate WB result then.

Neutral is very special.

Things to consider in a WB card:

18% gray cards are pretty dark, Even black "can work" but lighter gray or white can work better. Actual WB cards are white or light gray. The 18% cards are only specified to be 18% reflectance, they are not spec'd to be neutral. A $5 Porta Brace White Balance card is good.

There are many colors of off-white or off-gray to beware of, not close enough to neutral.

However, in the real world, many colors of white objects that "look like they should be neutral" are perhaps not all 100% perfect, but are in fact close enough for decent white balance results. White envelopes and copy paper, white T-shirts and shirt collars, porcelain dishes, church steeples and picket fences, white pizza signs, white plastic bottle tops, even white dots in the kids pajamas, all can mostly be quite close, esp so when no other choice. Wall paint or cars, not so much.