The Commodore 64's "secret colours"

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On the NTSC TV sets of the day, with a frame rate slightly less than 30 fps, changing the colors 50 times a second would produce odd artifacts rather than a smooth blend.


I remember the Amstrad CPC came with a 60hz monitor, maybe it was used on that system more effectively? I had one but don’t recall any discussion of it.

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They probably changed it 60 times per second on NTSC. I guess that would work, but with the interlacing, there would be some odd effects, especially at the edges of colors.


This is also how devs squeezed extra colors out of the relatively limited color palette of the Sega Genesis.

The trick worked because CRT displays use a phosphor coating on the front of the display, and it stays illuminated for a short time following the firing of the electron gun, hence flipping rapidly muxes two component colors together. Hook a genesis up to a modern LCD, and you’ll see the flicker clearly.




The first time I saw this was back on the Atari 2600 when rapid colour changing was used to give some objects a brilliant ‘sparkly’ appearance.

I just looked at the linked page, those colours look much more washed out than I recall - I’m sure there was a fairly good orange in the Commodore 64 - is this a case of the NTSC machine having a different colour space to the PAL version I played with?

As an apple ii user, I feel slighted.


You should. Those doubts you had about Apple back then we’re well founded. You should have gone down the Commodore path.


Exactly, the C64 lead to the mighty Amiga, but who even remembers Apple these days eh?


yeah, went from apple ii, to apple iigs, to various macs. Not a C64 or an Amiga in the bunch. I remember being jealous that the Amiga games tended to use the 12 bit palette to its fullest extent, while the GS games often received an EGA hand-me-down.

Pirates on an Amiga.

Pirates on a Apple IIGS


Ohhh, man! Be steady my anxious eyeballs!


How? That capability had nothing else to compete with within its own PC design.


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I remember doing pixel art on the Apple IIe, where high resolution meant only having four colors to work with. 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. Though I recollect it was mostly just slightly darker versions of a few of those colors…


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.

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

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“Dithering” was used too, of course. But I’m talking about something weirder and more idiosyncratic than that. I mean that when certain colors were positioned in certain places relative to other particular colors, one of those pixels would change to something other than the four theoretical colors of orange, blue, purple, green. If I remember correctly, orange could be made to turn a kind of olive green color and blue could be made a deep purple-blue, for example. It actually made dithering somewhat problematic, as it could create spots of undesirable colors in what should have been evenly shaded areas. I assume it was related to the effect where white pixels would sometimes look green or purple (depending on how they were positioned) because there would be a trace of that color on one edge that would tint the entire pixel*. As an effect, it worked best for pixel art, specifically for things like game sprites.

*The image below, for example, only utilized blue and white pixels:

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If you swap two colours rapidly enough - say at 50 or 60 frames per second


I remember thinking about this color strobing effect before I knew about things like 60hz refresh rates. It seemed like anything was possible. Oh to be young at the dawn of the pc again.

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