Every Color has a Value
Though we may not have thought about it before, every color has a value. By definition, only white can be 0% and only black can be 100%. Merely choosing colors for type that are very different in hue is no guarantee of legibility. It still comes down to value. A designer needs to ignore hue and saturation when selecting colors for type over colored backgrounds. Instead, designers need to learn to consider the inherent value of each color to ensure there is sufficient contrast for good legibility.
Special note: If excellent contrast is 60% differential or greater, it is impossible to achieve excellent contrast over a 50% background.
The Dreaded V Words
If a background and any type over it are close in hue and in value, the type will vanish. Here magenta type is on a red background.
If both the background and type are very different in hue but close in value, then vibrating will occur. While this is more legible than vanishing, it makes up for it by being irritating and should be avoided at all costs. 
As with other issues created by values, these are made worse when type is small or being viewed at a distance, such as in signage.
In the book covers shown below, we can see that the red type on a blue background is difficult to read in the title and nearly impossible to read in the smaller type. If you browse any online bookseller you’ll see how often book cover designers make this mistake, and not just the self-published ones, either.
This contrast issue is made worse the smaller the type is. Now that the majority of books are bought online, the first time most book covers will be seen, they will be only about 150 pixels high.
Difference in hue has very little bearing on contrast; only value provides it. And the damage is worse the smaller the type is or if it is lighter in weight. If you think this isn’t a big deal, just try to view these covers from even a few feet away.
Busy Backgrounds
Type frequently needs to be placed over a photo or other imagery. If the background for type is busy, the type may be impossible to read. Busy backgrounds are defined as those in which both light and dark elements exist. No color of type will be easy to read over a busy background. Remember, the question is not, “Can the viewer eventually puzzle out what is written?” but rather, “Is this design helping or hurting communication?”
The text says, “What color type can easily be read over a very busy background?” The answer: Nothing will work. Don’t put text over a busy background. Ever.
This doesn’t mean that a background must always be a flat color. Just make sure that the values are either all light or all dark; then surprinted or reversed type will have a chance. However, if parts of the texture are in the mid-value range, it may be impossible to achieve excellent contrast (60% difference or more).
Question: What if the photo doesn’t have either all dark texture or all light?
Answer: Time for an assist from Photoshop. 🙂
The text above says over the water, “Even fairly small type can work over a non-busy background. Thanks to Cocoparisienne at Pixabay for this image.” Over the buildings it says, “Over a busy background, one with both light and dark, no color of type works.”
Widely Varying Backgrounds
A similar problem occurs over widely varying backgrounds, such as over a gradient that has light, mid and dark portions in its progression. Again the answer is: nothing will work well. Some parts will survive, maybe even have quite good contrast, but others will be illegible or at least compromised. Don’t do it.
A widely varying background is one where both light and dark values are found. No single color of type can give consistently good contrast over such a background. Some part is sure to have poor contrast.
Does that mean that all gradients are useless? 
No. But if text is to go over it, then limit the background gradient to the light third or the dark third of the value scale and make the type the other value extreme. For instance, make the background gradient values from about 66% to 100% with light type over. Or else make the background gradient from about 35% to lighter with dark type over. Remember to avoid colors with equivalent values near 50%. It is impossible to get 60% away from a background with a value of 50%.
Adapted from Logo Design Theory: How Branding Design Really Works