These colors combine to create a color space called RGB . As Web designers we are concerned with this RGB color space. CMYK colors are used by print designers. Ironically, you will not get better image quality with higher resolution when designing web pages. Also set your graphics to be measured in pixels , not inches.
Pixels are the standard measurement for screen-bound images.
This can have a huge impact on file size. At a distance the human eye merges these dithered patterns into another color, but up close the image appears speckled. Because different operating systems and applications build color palettes according to their particular needs, color variance arises when moving across platforms and programs. You should prepare for the fact that visitors to your website will be using different systems with various color settings. There is, in fact, no one universal 8-bit palette that will not dither on all platforms.