The Commodore 64's "secret colours"

[redesigned]

@Shuck

One quickly realized that if you positioned pixels of certain colors in particular positions relative to other specific colors, it created a few extra shades.

This same trick was later used to get 16k colors from the 256 color space, both for older monitors and for web images. The web used to have a 256 color safe pallet that made sense mathematically but was horrible visually. Tools such as ColorSafe made it so you could get full color photos as well as any color you wished to display using only the 256 web safe colors.

ColorSafe is a filter plugin for Adobe Photoshop and compatible applications that provides fully interactive color pickers for the easy creation of almost limitless hybrid colors from the base colors in the Web safe palette. A hybrid color is formed by combining four pixels of two different colors from the Web safe palette together in an exact ordered pattern to create a third visually distinct color that will not dither across viewing platforms and monitor depths on Web pages.

Same company (BoxTop) made ImageVice, PhotoGif, SuperGif, PhotoHTML, etc.