What a fake invoice can teach about color perception and graphic design best practices

Below is a fake invoice that was sent out in the hopes that the receiver - likely a secretary in a large business - would pay, since it appears legitimate and is for a relatively small amount.

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Note that all the commands to pay use colors that have a strong tonal contrast with their background – white against dark blue, black against light blue or white. However, the disclaimer that reveals that this is not an actual invoice but a solicitation to accept an advertising offer is printed using color contrast rather than tonal contrast – yellow lettering against a blue background.
The designer of this "invoice" has effectively leveraged an aspect of human vision that puts a priority on tonal contrast rather than color contrast. By making the tone of the yellow disclaimer virtually identical to the screen tint of the blue background – the text effectively disappears, or at best becomes very hard to see/read.
Less sophisticated designers usually, and wrongly, believe that color contrast is a way to make text stand out from its background, i.e. they typically believe that the color contrast of yellow against a blue background should make text stand out and be more readable.
Interestingly, the give-away, in this example, is that a slight misregistration of the yellow text against the light blue background has increased the contrast of the letter edges slightly and made the disclaimer, just barely, more visible/readable.