Because not all K's are created equal. OK?
You all know about the Kelvin scale of colour temperature measurement - named after Lord Kelvin - that measures the relative redness, orangeness, yellowness, blueness, and eventual whiteness of light and strings it out on a numerical scale. The scale starts at 1, which is the colour and intensity of a mechanic's fingernails and goes to 10,000 which is he whiteness of sunlight as you come off a three-day bender and someone throws the window blinds open. I suspect Kelvin also had a scale for the screams as well.
Well, a lot of cameras skirt around this question by having automatic white balance programs in their software that measures the light, quantifies it on the Kelvin scale, and then gives you whatever the heck it wants to. If you include a dominant colour in your composition, it sometimes agrees with you and renders it accurately and sometimes argues the case by tipping the white balance over the other way to compensate for what it thinks is your error. If you are easily persuaded, you go along with it. Hey, it may not be science but it might be art...
If you are pedantic and scientific and people have started to get wise to your pictures of snowdrifts that look like a sort of a dirty peach colour*, you override the manufacturer's opinion and set the white balance yourself...by following the manufacturer's preset WB options. Now you can get a light blue or green snow drift. If you snow drift is lit by a fluorescent tube you can get just anything...
Determination is the thing. Your camera probably has a custom setting for the white balance in graded steps of Kelvin degrees. Find it in the menu next to the pet smile option and dial in 5000ºK. If you are out in fictitious sunlight at 11:00 in the morning with the sun to your back and a chocolate-box landscape stretched out in front of you you will probably get a reasonable balance. Likewise if you are in a studio with a studio flash popping away at a X-Rite colour card held by someone who is a neutral colour themselves...you'll get something recognisable.
But - modify that old sunlight with cloud, haze, chemtrails**, or advancing hours and it will get red or blue or orange or whatever - also if you are shooting in the studio you may discover that your beloved strobe units don't fire at 5000ºK.
I know - My studio lights fire at 5300ºK. Not a big difference, but enough to throw off some grey backdrops and some delicate colours. When I noted this happening I conducted a long test and look day and finally got the right number. I have it dialled in on a custom setting on the Fujifilm cameras and the whole thing now works a treat.
If you can't afford a big X-rite or an old McBeth chart get a Spyder ColorCheckr24 and make that do - you'll be glad you did in the end.
* Note peach coloured snow was common as winter wore on in Alberta. And there were worse colours as well...
** When I was a kid our chemtrails were maple syrup...
