A brand guide ought to be as solid as possible regarding colors given that the whole purpose of the guide is to provide a basis point for decisions and specifications to use to ensure some semblance of compliance going forward.
That being the case, all the advice you’ve received in the other posts is smack dab, right on accurate.
However, it’s often not quite that straight-forward.
If you’re dealing with a company where their logo color needs to match exactly with their trade show booth, which needs to match exactly with the screen printing on the t-shirts of those in the trade show and where both must match exactly the color on both their slick, shiny hand-outs as well as on their uncoated paper business cards, it’s another matter. In those cases, you’ll need to get things exact or, at least, provide a color standard that can be used by vendors to ascertain what needs to be done by to match those colors.
On the other hand, there are companies where those in charge can barely tell the difference between dark red and bright red and who think all the fuss over colors is just a secondary, nuisance kind of importance. In those instances all that attention and time spent to make everything exact might be in vain and never used.
I recently completed work for two separate companies: one was a construction company, the other was a software company.
The construction company had an MBA leading their marketing efforts and, like many MBAs, he knew just enough to be dangerous, while discounting most everything he didn’t know as irrelevant. He was fixated on everything the company did being colored Pantone 3125. I was designing a set of promotional booklets for them and, of course, Pantone 3125 or its CMYK approximation needed to figure prominently. The MBA had no idea of how color worked, but had that 3125 number memorized. There was not a conversation where that number didn’t come up several times. All their construction trucks were painted the equivalent of Pantone 3125, and he sent me photos of them for the booklets.
Of course, all the photos were a bit different. Some of the trucks were in bright sunlight, some were in shade, some were shot on cloudy days and, as to be expected, the colors of all those trucks were all a bit different depending on the photo. He was upset about this and wanted each of the trucks in each of the photos retouched so that each matched exactly to Pantone 3125. He didn’t understand that the same color indoors is actually a different colors when seen outdoors or on a sunny day as opposed to a rainy day. I probably wasted 500 dollars of their budget demonstrating to this guy how awkward his photos would look if I shifted the color balances on all their trucks in all the photos to match up precisely with the Pantone 3125 swatch taped to his white board.
Their style guide, which I got a copy of, mentioned everything from logo placement, to typography to most everything else a typical style guide gets into. He didn’t care about the typography though. He couldn’t tell the difference between one sans-serif typeface and the next. But for whatever misguided reason, his eyes were perfectly attuned to spotting even the slightest deviation from Pantone 3125, even though he didn’t have the slightest idea of anything having to do with color theory.
The second company — the software business — has a CEO who knows just enough about design to appreciate that he knows almost nothing about it. The company is composed of researchers, engineers, mathematicians and hard-core programmers. A photo of a typical company employee could be used to illustrate the word “geek” in a dictionary. Most any design decision turns into a vote by the employees.
They have some commercial products, but marketing is a complete mystery to them. Brand consistency is a buzz word they’ve learned, but fail to understand. The logo I designed for the company is blue, so when they had company t-shirts made, they bizarrely chose to print their blue logo on a slightly different blue shirt. When I asked, their explanation was that they all took a vote and the majority wanted aqua-color shirts, despite the logo being a solid blue with no hint of green in it. Everything I do for this company is an exercise in bringing some semblance of order to their visual chaos.
A few months back, I suggested to the CEO that they might hire me to put together a style guide for the company to provide a little guidance. He agreed that would be a good idea. I carefully figured out all the colors, as suggested by those earlier in this thread. I wrote a 40-page brand style guide that covered everything from their website to their software interfaces to their promotional materials and PowerPoint presentations. I’m pretty sure it just went into a drawer someplace and never looked at again because everything I see they’ve done recently is a mishmash of random piecemeal stuff that’s sort of half made up with only a token bit of my brand guide being apparent.
This and other experiences accrued over 40 years of doing this kind of thing has taught me that every situation and every client is different. For some, strict adherence to Pantone and/or LAB colors is important and needs to be part of the process. For others, it can be a wasted effort when all they really want are the hexadecimal colors because it’s close enough for whatever they have in mind. For them, just keep everything in the CMYK gamut and hope they don’t screw it up too badly.
It’s important to figure out what the company really wants, really needs and temper that with a realistic assessment of how they’ll use it, what they’ll get out of it and whether or not it will be something they will take to heart or, to some degree, ignore. Then, with all that considered, give them what seems appropriate for the money they’re paying you.