Hi,Im new to this field kindly tell me that what is the difference b/w 2 color offset or 4 color offset.

every answer would be appreciated

More and more jobs are going to process color (cyan, magenta, yellow, black) for full color output vs. spot color accents. Thus the major difference is that with a 4 color + offset press you can adjust registration and ink densities all at once and actually see the final result before the whole run is printed. With a 2 color press you have to run process color in two runs; with a 1 color press you have to run process color in four separate runs through the press. With a 1- or 2-color press you register by the registration marks and adjust ink keys using the color bars to run "by the numers" so to speak but you won't actually see how the image, photograh, etc being printed looks until the final pass. (and you will only have the colors of the final run to adjust if you find you're not matching the proof well) If everything in your workflow is accurate, it's no problem, but if not, running on a 4-color press allows you to adjust to the proof before the full skid of stock has been printed. And of course, it's faster: less individual registration and density checks, less or no downtime for changing ink in the press, less moving of paper, etc.
But as the 2-color presses are being upgraded to 4+ color more modern presses, there are some deals to be had if you have some extra time for them.

isn't it very difficult to understand that how these presses works
Well,I want to start a press to print books and magazines in good colors,so what I know is that webfed is used for them.Still cant decide whether to purchase 2 color offset or 4 color.which one u preferred to print about 1 million pages per month
waiting for reply,thanks

The shops that are buying 2-color presses nowadays to print 4-color are those that don't have a budget of $75,000+ for a 4-color press.

so what is the cost of 2-color presses SM-printer

There are press brokers in the forum here that can give you much more concrete and specific numbers, but if you have time to wait for the press you want to become available you can find an older Heidelberg GTO 46 2-color in the $25,000 to $30,000 range.