Codec 21 is not a font as such, is a content coding system (not necessarily textual) that uses the principle of light colors (like it can use any other). It’s based on the difference in position between three overlapping grids.

Three lines of text are shown, each with one variation of Codec 21 (R, G and B)

When the lines overlap, the visibility of messages decreases.

When this same principle is applied over one color text (black, for example), such message becomes unintelligible. It would requiere an identical but inverted pattern to be decodified.