If you mean exactly the same, compare each pixel.
If you mean compare a RGB image and a greyscale image, you need to convert the RGB to greyscale first, for doing this, you need to know how you did RGB->Greyscale before, there're different ways of doing this and you could get different results.
Edit, if the method it used in RGB->Greyscale is liner, you could work out a,b,c in the formula grey = a*R + b*G + c*B by comparing 3 pixels.
