What we call Black/White photos are in fact greyscale images. Photographs (color and greyscale) are continuous tone images, so called because the photo, unlike a digital image, isn't composed of square pixels but rather continuous areas of different colors or shades. To represent this information in a digital format requires dividing the image into pixels using 8 bits of information for each pixel, producing 256 possible shades of grey. The shade of each pixel ranges from a value of white(255) to black(0). Greyscale is used for many other things besides black/white photos. When you learn about masks, you will find out that most of the masks used in photo-editing are actually greyscale images. With the explosive growth of internet, 256-color images have become very popular. With the exception of some Windows and Web page icons, 4-bit is rarely used, so we will devote most of this discussion to 8-bit or 256-color images. Reffered to as paletted or 256-color, an 8-bit color image can only have one of a possible 256 combinations of color assigned to each pixel. This isn't as limited as you might imagine. When an image is converted to 8-bit,the image-editor creates a reference palette to which all of the colors used in the image are assigned - hence the term palette. Many people are under the impression that 8-bit color is markedly inferior to 24-bit color. That used to be true, but the process of converting the image from 16 or 24 bit to 8-bit color has been so dramaticly improved that in any cases it is difficult, if not impossible, to tell the original from the paletted one. The 16-Bit color depth reminds me of the EGA monitor standard. There was a brief time when CGA wasn't enough. - CGA = Color Grafics Adaptor, EGA = Enhanced Grafics Adaptor.- There is nothing worse than seeing a graphic computer game in CGA, and so EGA came next, It offered more colors and slightly better resolution than CGA. EGA was quickly replaced by VGA (Video Grafics Adaptor). In a way, 16-Bit color is like that. It came at a time when 24-Bit color was just too expensive and 8-Bit wasn't enough. Using 16 bits to define the color depth provides 65.536 colors This is enough for almost any color image. What are the advantages of 16-Bit color ? Faster performance because you are moving 1/3 fewer bits. When will you use this? You may see it if you have a limited amount of video RAM and you increased your resolution settings. True-Color images may use up to 16.7 million colors. They are so closely associated with RGB color model that they are sometimes reffered to as RGB 24-Bit. RGB stands for Red-Green-Blue. Your monitor makes all of it's colors using a combination of these 3 colors. your eye perceives the color the same way. The 3 colors that make up the RGB model each have eight bits assigned to them, allowing for 256 possible shades of each color. Your monitor creates colors by tracing the image on the inside of the monitor's glass tube with 3 electronicly made lightbeams, called cannons. Each color-cannon in the color set of 256 combinations produces a possibile 16.7 million color combination. While True Color doesn't display every possible color, it gets pretty close. It is the color model of choice for the computer artist. Look back at the color depth table, do you notice anything unusual about 32-Bit color? Although the color depth is increased by 25% over a 24-bit image, the number of colors remain the same. Why is this ? There are 2 answers,because there are 2 types of color depth that involve 32 bits. The first is more commonly seen on the Mac side of the world. When they say something is 32-Bit, they are refferering to a 24-Bit RGB model with an additional Alpha Channel (here we go again on that Alpha channel,hehe). Apple reserved the alpha channel, but it has never specified a purpose for this data. Alpha channel has come to be used by most applications to pass the greyscale mask information. The other 32-Bit type of color images expresses a CMYK model. C = Cyan, M = Magenta, Y = Yellow, K = Black (because B used for Blue). NOTE > Most of the grafic processors are advertising that they offer 32-Bit, 64-Bit and even higher, grafic processor boards. This has nothing to do with color depth.It is a reference to the width of the data path. The wider the data path, the greater the amount of color data that can be moved, and therefore the screen is redrawn (refresh rate).