I am looking to design in color mode 'bitmap'. I don't understand the use of the color mode bitmap. When and how can I use it?
It can be said to be an (old) alternative to vector. I have used it for some very old logos and drawings, but more recently in signatures. It is not vectorized, and not anti-aliased, so it needs to have very high resolution in order to not appear pixelated.
It can only have one color. If you place a bitmap colored file in InDesign, you can actually change the color there. Which is quite handy especially for signatures.
This is kind of a history thing, I'm sure the mode is being used in creative ways differently that how I describe, but ---
In traditional PostScript-based offset (big printing press) printing the imaging is done in one of two ways (to simplify things) --
As a half-tone where a raster image is broken down into dots and those dots are in a grid with sometimes limited resolution (say 85-110 of these half-tone dots per inch).
As individual "pixels" printed at the device resolution (which could be 1,000+ "pixels" per inch, depending on the device).
Continuous tone images (photographs) are normally rendered using the half-tone process, and items like text are normally rendred using the individual "pixel" approach -- with text (and vector art) taking an intermediate step of being represented as math at first. That keeps the text sharp, but limits how you can use color with text without using tricks.
The Photoshop bit-map mode was used mainly when scanning in line art (like old-school black and white cartoons that are drawn in black ink, with no grays, or black and white logos), it allows you to print the black and white artwork with as much detail as the printing device allows, without going through the half-tone process.
Black and white cartoons for newspapers are still produced this way. In fact, the color cartoons are normally the bitmap mode black and white scans with colored half-tone raster overlays (probably designed in Illustrator and then rasterized, but I don't know for sure).
If you have a monochrome high resolution image >900 intended for print. Then you can save it as a bitmap. Bitmap images can be very aggressively compressed making the file size very small despite having a lot of pixels. Only realy useful for printing, or archiving paper documents.
Now the thing is a bitmap image more closely works like a printer does and not like a monitor does and as such it contains only one channel and 2bits of information. This can be useful if you do something really really special, like doing your own rasterization (for example you make your own stochastic screening). You can also use bitmaps as sort of a replacement for vector images for scanned content.
Its not a new versus old technology thing as others have suggested. Its just rarely used these days, since your computer is probably equipped with a few gigabytes of memory, if not quite hundreds of gigabytes. And looking from the other end your not doing so special things with your printer that you need it for that.