Custom palettes and system palettes

According to your screen resolution, you can see up to 256 colors, up to 65536 colors or up to 16777216 colors. It depends on the monitor and the video board. For example, my screen resolution is: 800x600 pixels and 65536 colors. Whenever I attempt to increase colors (up to 16777216) the operating system decrease the screen resolution to 640x480 pixels automatically. Consider that today the average screen resolution is 800x600 pixels. But things can change (time ago the standard screen resolution was 640x480 pixels). Anyway, you have to know something about strange things that happen when the screen can handle 256 colors only (8 bits). In fact Mac and Windows operating systems use different system palettes. Mac systems have 2 reserved colors: black and white. You can't change them. Other colors are free, and you can change them as you prefer. On Windows systems on the contrary, the first colors and the last 10 colors are reserved. The other 236 colors are free. Anyway, these are the default system palettes, but you are not forced to use them. In fact you can use your customized palette. If you use your customized palette, you have to ask a question to yourself: how many colors can be handled by the screen used to display your GIF? Problems come out when your GIF has to be rendered on 8 bit screens (screen set up to display up to 256 colors). In fact these screens can display one palette only: you can't use more than one customized palette at the same time. For example: suppose you are building 2 different GIFs: the first one containing all shades of blue (e.g. an expanse of sea), and the second one containing all shades of red (e.g. a sunset). Now suppose you put both of them on your web page. What happens? Simple: the operating system uses the FIRST ONE palette found on the web page, displaying your sunset with the colors used on the first GIF (shades of blu). Well, it isn't really so, because the web browser uses ITS OWN palette, taking not care to YOUR customized palette. In other words, the web browser uses 216 colors: the colors which can be rendered on any monitor and any operating system. This is the so-called 'safe palette'. But the web browser can't understand what color is better to replace one other. For this reason if you use the safe palette (instead of your customized palette) from the beginning, you can modify your GIFs as you like, avoiding the problem caused by the web browser's remapping procedure (press the SHIFT key and click HERE to download the 'safe palette'). Anyway remember there are not problems when your visitors use a screen that can render more than 256 colors.