The image an 8-bit gray scale while another is a colored image.
i want need to send 2 bits of the gray image in the red channel, 2 bits in the green channel
and 2 in the blue channel of the colored image.( i am nt getting it can any1 help me to start , i havent done image processing tools in matlab )

 

You could truncate the color image to 6bits per color and use the now free lowest two for storing your grayscale image. I don't think this would cause significant distortion and I would love to see the results.
In what environment will you implement this?

 

You could add (with rollover) something like a pseudorandom number to each pair of bits. If the sequence is always seeded the same then you can subtract the same sequence at the end to get the data back.
This way, even if the colour image has a patch of plain colour, there will be nothing but noise added.