what is the need to convert an image into binary image in detecting colour using image processing?

is it necessery to convert the image into binary for detecting the colour bands in that image(eg:resister)?

[Guillaume]

I would suggest you grab a book on image processing. It's a better way to learn about it than asking questions on internet forums.

If you want to detect colours, then binarising an image is completely absurb since by definition binarisation reduces the number of colours to just two.