Legibility is readability, the capacity to be correctly perceived or clearly deciphered. Legibility is a function of contrast. And contrast is a function of value. It doesn’t matter so much what the hue or saturation is in the colors. What matters most for contrast is sufficient difference in value.
Any logo that has low contrast – and therefore low legibility – has failed its very reason for being: to be clearly seen and read.
There are two major kinds of contrast, and they are both essential for a good corporate identity: external contrast and internal contrast.
External contrast means having a good value difference between the design and the background. Internal contrast means that the logo elements can be distinguished from one another. A good identity has both.
This is another drawback to using gradients. In one area there may be sufficient contrast, but in others not.
One recurring pitfall that some designers fall into is trying to use all three of the primary colors in an identity. It never works. Yellow is a very light color, and consequently will not show up well against a white background. Most shades of blue are too dark to show up against a dark background. Red is usually a mid-value color and will not show up against a mid-value background. Therefore, any identity that uses all three primary colors risks having some part fail to show up, no matter what color background is used. It is one of those “great ideas” that isn’t great after all.
As we discussed before, it is helpful to think of the values of colors as percents. 100% as black; 0% as white, etc. Since a signature needs to be easily read to be effective, it needs to have 60% contrast or more with its background. A logo can have less contrast, as little as 40% with the background. But if a logo has multiple colors–not something that is recommended in the first place–then the elements inside need to have 30% or 40% contrast from each other to be effective. As you can see from the grayscale version of the illustration above, every one of these fail completely at this.
A designer can sculpt the most exquisite shapes with the best proportions and then ruin the design by choosing colors that give poor contrast.
Sounds like a blowout, doesn’t it?
Adapted from Logo Design Theory: How Branding Design Really Works