A style guide is a reference sheet that defines the visual aspects of your brand, such as your logo, fonts and colors. It’s an essential tool for maintaining consistency throughout all of your branding materials, and is particularly helpful if you’re working with an outside designer or printer, or have multiple people creating new things for your brand. It can also help save time, so you’re never scrambling to find a new font or color to use.
Style guides (also referred to as "brand bibles") can get really in-depth and cover everything from your mission to your target audience, values and brand personality. Below we've outlined some of the more basic, visual design-related to include.
1. Your logo
Consider all the ways it might look in different places, and include multiple versions if necessary. For example, you might have different versions for your website homepage, business cards, letterheads, etc. You should also include any alternate color options you have (reversed, black and white, etc.).
WhatsApp offers several options of their logo to use for various layouts and occasions. (View the rest of their brand guidelines here.)
Specify the minimum size your logo should be displayed at, and whether it should be surrounded at a certain amount of empty space.
2. Your colors
This will typically be the colors from your logo, as well as a few complementary colors. This section should include HEX codes for web use, and CMYK values and Pantone colors for print. Conversions from RGB colors (using HEX codes) to CMYK can be dramatic sometimes, so be sure to test all of your colors. (Learn more about the differences between RGB, CMYK and Pantone colors.)
Coolors is a great, easy source for generating color schemes. You may also want to try Adobe Kuler and Paletton.
3. Your fonts
Specify which fonts go where — such as which fonts will be used for headings, subheadings, body text, etc. You can also specify sizes, weights, styles, etc.
from Mailchimp's style guide
4. imagery
Include any graphic or web elements and icons you might use. You might also want to describe what style of photography should be used, and if there are any visual elements that should be avoided.
Skype's style guide includes its own illustrations