Would it be possible to set the slideshow so that the images fade up from black and white to the full colour original image as one of the fade effects?
I don't know if this is something that Monoslideshow can do for you but, with other packages, I do this by creating 2 copies of the image, one colour and the other black and white. Add the B&W image immediately before the colour version of the same picture then fade from one to the other.