1. Sega Genesis
The Sega Genesis usually uses a 320x224 graphics mode. Some games use a 256x224 mode. This mode's horizontal resolution is so great that not all the pixels can be fully resolved in a composite video signal on a standard TV. There is frequent color fringing in thin-font text through the composite signal. Additionally, with alternating colors, you can obtain color artifacts to give graphics a transparent effect. This effect was used fairly often and works best with long alternating vertical lines.
Unfortunately, this effect is lost with an RGB monitor. Compare the Genesis screenshots on this page for examples : http://www.chrismcovell.com/gotRGB/screenshots.html
Here are some examples from several Sega Genesis games. For these screenshots I am using Kega Fusion v3.64, with the normal RGB output and after the composite filter has been applied. Although the composite filter is less aggressive in blending pixels on a real machine, it does illustrate my point well enough. In each pair of Genesis screenshots, you will see the RGB first, then the composite.
Earthworm Jim is particularly ugly looking with RGB :
 |
 |
The waterfalls of the Sonic series just don't look as convincing in RGB, composite video gives them a transparency effect :
Also, compare the fronds of the palm trees in Sonic 2 :
Some games use a checkerboard pattern that allows for dithering to give a transparency effect. It is not quite as seamless on a composite monitor as the vertical stripes method, but gives a more acceptable picture on an RGB monitor. Streets of Rage 2 uses the effect to simulate the characteristics of a studio spotlight during the nightclub stage :
The effect looks obvious with RGB, as do the color limitations in this title screen for Echo the Dolphin :
You can even see dithering being used in Street Fighter II, which uses a 256x224 mode, to smooth out the color gradient's in the carpet in this stage :
However, sometimes composite video can produce some very unsightly artifacts, as shown in the empty life bars of Castlevania Bloodlines :
On a real NTSC monitor, you will see color fringing between the white rectangles of the life bars.
2. Super Nintendo
The Super Nintendo almost always uses a 256x224 graphics mode. The Turbo Grafx 16 and CD typically used a similar mode. Thus for games using these resolutions, artifact graphics are not typically available. Even in these systems, shadows and smoke/fog do make some us of the less-than fine resolution of composite video :
However, late in the SNES's lifespan, Nintendo sought to improve the graphics quality of some of its games by creating 3D models of sprites and background tiles on advanced Silicon Graphics computers and then storing what was needed in a pre-rendered form on the cartridge. Pictures of these graphics looked awesome on boxes and manuals and magazines.
Games with this look, like the Donkey Kong Country series, Killer Instinct, Super Mario RPG were very popular and helped extend the life of the SNES without a silly CD add-on. When playing the game, they looked amazing back in the day. However, when played through and emulator or to a lesser extent through RGB, the flaws cannot be denied. The graphics have been so reduced in resolution from their SGI originals that they tend to look fuzzy, even with the perfection an emulator like higan 0.94 can provide :
Composite video can help hide the rough edges from the down-conversion.
The SNES could do true transparency, but even so, dithering was sometimes used to provide something akin to free transparency. The first screenshot, from Chrono Trigger, shows natural transparency by the light streaming in from the window :
The second screenshot, from Secret of Mana, shows true transparency with the water covering the rocks, and dithered transparency in the text box :
The final screenshot, from Mortal Kombat 3, shows a transparent effect with the life bars :
To give a flavor of more accurate dithering, I used SNES9x 1.53 with blargg's NTSC Composite filter for all the SNES composite screenshots.
Occasionally, Genesis-like artifacting does appear on the SNES. Consider Kirby's Dream Land 3, which uses a 512x224 resolution. The increased bandwidth of an RGB monitor can essentially resolve 512 pixels, but a composite monitor cannot, and the result is free transparency on the lower quality device. Compare the following :
