When changing or modifying some background colors for the web...I noticed the following differences in various examples:
When I look at the jpg or whatever magnified in FW CS4 magnified maybe at 1600...the pattern is actually a number of differing pixels or squares as follows:
But my fills are always just solid color as follows:
I hope these show up online to show what I mean. What do I do to fill an object with a color that is made up of a number of similar colors (1st example) rather than one solid blob of the same color (second example) ?
The squares in the "solid" color .jpgs are the artificats of the JPEG compression scheme.
The way that algorighm works is to make a mathematical construction of how the luminance and color information in your image changes and then throw out the high-frequency information. ("High frequency" means places where the colors vary dramatically from one pixel to another, such as a black line on a white background.) The algorithm works in 8-pixel blocks. Because data is thrown away (the compression scheme is "lossy"), saving as a .jpg introduces noise and color shifts.
JPEG stands for Joint Photographic Experts Group. The format is most suitable for photographs and other images where the data has slow variations. As you've noted, the format is not well-suited to images where there are large areas of solid colors and line art. For line art, the GIF and PNG formats are better suited.
If you want a noisy image, such as your red example, use the filters to add noise, export as .jpg with low quality, zoom in, make screen capures...repeatedly.