Do I need brand guidelines?

The reason to write it down is not aesthetic discipline. It is that the drift happens through retyping and sampling rather than through anybody deciding to change something. Somebody produces a graphic and picks a colour from an existing image, which is already compressed and slightly off. The next person samples from that one. Six months later nothing matches and nobody made a decision.

Record the colours precisely and in both systems. Hex values for anything on a screen, and the print equivalents obtained from a printer rather than converted yourself, because screen colour and print colour are different systems and a vivid screen colour frequently cannot be reproduced on paper. Getting the print values at the same time as the screen ones saves discovering the problem on a delivery of business cards.

Name the fonts and say where to get them, including which weights you use. A document produced later by somebody without the font will substitute something approximate, and that substitution is the second most common source of inconsistency after colour. If the fonts are licensed, note the licence terms in the same place.

Specify which logo version is used in which situation, because that is the decision people get wrong when improvising. Full colour on light backgrounds, single colour on dark, the mark alone for profile pictures and favicons, and minimum sizes below which it should not be used. Include the actual files alongside the page rather than describing where they are.

Include the standard description of the business, in one sentence and one paragraph. This is the part almost nobody includes and it is used more than the visual elements: directory listings, social profiles, proposals, and anywhere somebody has to describe what you do. Having an agreed version means all of those match, which matters for local search as well as for consistency.

Keep it to one page precisely because longer documents do not get used. A twenty page brand manual for a business of one is a document that exists rather than one that is consulted, and the entire value here comes from somebody actually opening it before producing something.

Store it where the work happens rather than in a separate folder, and put the source files beside it. A guideline nobody can find while designing is a guideline nobody follows, and the friction of hunting for the logo is what produces the version pulled off the website at the wrong resolution.

Use it as the source rather than working from an existing file. Sampling a colour from a previous graphic propagates whatever error was in that graphic, which is how brands drift a shade at a time. Every new piece starts from the recorded values.

Then hand it to anybody who produces anything on your behalf, including printers, sign makers, and contractors. Each of those will otherwise interpret your brand from whatever they were sent, and the page is what makes four suppliers produce work that matches.

Include a short do not section, which is the part that prevents the most damage. The logo should not be stretched, recoloured, placed on a busy background, or reconstructed by somebody who could not find the file. Three or four explicit prohibitions catch the failures that occur when somebody improvises under time pressure.