Contrast, and how a label fails
On this page8
The requirement
Conspicuous and legible type, in contrast by typography, layout or color. Three routes to the same end, and a label needs to achieve it by at least one of them.
The named failure
Color combinations lacking sharp contrast. Black on dark blue is the example that gets cited, and it is exactly the kind of choice a designer makes to keep a warning from dominating a package.
Obscured or rendered inconspicuous
Cautionary material obscured or rendered inconspicuous by design elements, vignettes or surrounding text misbrands the product. The standard is about the effect on a reader rather than about any single measurable property.
Which makes it the hardest rule to self-assess
Type size you can measure. Separation you can measure. Whether the warning reads as a warning against a busy photographic background is a judgment, and the person who designed the label is the worst placed to make it.
Consistency within the hazard statements
All the hazard statements have to match each other in size, style and color or boldness. A second hazard set a little lighter than the first, for balance, breaks that.
A practical test
Print it, put it on the shelf among competitors at normal distance, and ask somebody who has not seen it what the warning says. If they have to look for it, the standard is not met regardless of what the ruler says.
And test it on the actual substrate
A warning that contrasts well on screen can disappear on a clear label applied to an amber bottle, or on a kraft stock. The substrate is part of the color decision and it is usually chosen after the artwork.