Composite artifact colors is a technique commonly used to address several graphic modes of some 1970s and 1980s home computers. With some machines, when connected to an NTSC TV or monitor over composite video outputs, the video signal encoding allowed for extra colors to be displayed, by manipulating the pixel position on screen, not being limited by each machine's hardware color palette.

This mode was used mainly for games, since it limits the display's effective horizontal resolution. It was most common on the IBM PC (with CGA graphics), TRS-80 Color Computer, Apple II and Atari 8-bit computers, and used by the Ultima role-playing video games. Software titles (such as King's Quest for the IBM PC) usually provided an option to select between "RGB mode" and "Color Composite mode".

On PAL displays the effect is also present, but generates more limited colors. Depending on the exact PAL system used results will vary (if PAL-M or PAL-N are used, color artifacts similar to NTSC might be possible).

Although related, artifact colors are not the same as horizontal blurring. Blurring is a general effect of using a composite connection, that simply creates new colors due to a mix of adjacent horizontal pixel values. The exact mix will depend on the saturation and specific colors of the original pixels. Nevertheless, this effect can be exploited by using dither patterns, generating new intermediate palette colors on machines with a sufficiently high resolution display, like the ZX Spectrum, Mega Drive/Genesis, NES/Famicom or Amiga.

In the NTSC color system as used in broadcasting, the color subcarrier frequency is exactly 227.5 times the line frequency, i.e., each line contains 227.5 color subcarrier cycles. This causes the apparent phase of the subcarrier to be reversed every line, which results in solid colors being displayed as a checkerboard-like pattern when viewed on a monochrome display that does not filter out the color information.

Computers such as the Apple II and the CGA video card for the IBM PC, output a signal that is an approximation of the broadcast standard. In both the Apple II and the CGA, each line is elongated to full 228 cycles of the color subcarrier. This is within the tolerances of most displays, so the image is displayed clearly, but the pattern generated by solid colors becomes straight vertical stripes instead. Each horizontal position within any line has constant phase relationship to the color subcarrier under this system, so lighting up a pixel at each specific horizontal index always has the same effect on the color information as interpreted by the display.

It is also typical for these types of display adapters to have pixel clocks that are a multiple of the NTSC subcarrier frequency. Both the Apple II and the CGA use the pixel clock of 14.318 MHz, four times the color subcarrier. For a broadcast-quality signal, that would mean 910 pixel cycles per each line (as opposed to 858 as later standardized by the ITU-R Recommendation BT.601), with about 750 of them occupying the visible portion of the screen. With the stretched lines of these early computers, each line was actually 912 pixel cycles long, and only a portion of the visible area was used - 560 pixels in case of Apple II (although not individually addressable without an 80 column expansion card), 640 in case of CGA. Each pixel could have one of the 4 predefined phase relationships to the color burst, so a "fake" subcarrier that will be interpreted as color by the display, can be constructed by outputting specific pixel patterns.

In case of adapters that also have native color capabilities, such as the CGA, this technique can be further expanded by forming patterns out of the built-in colors - this way, the "real" subcarrier generated by the hardware will interfere with the "fake" one residing within the pixel patterns, causing the display to interpret the result as new, unique colors.