Dave

hi pursedog mate you should really have checked the substrate for any movement and secured it down before tiling, how long was it before you told customer they could walk on floor, if walked on to early this would have caused the inital cracking of the grout, as for differnet shades in the grout did you mix grout properly, i.e correct ratio of powder to ratio of water as defined on grout bag, mixing to dry could also have caused this... and also did you fill joints when grouting leaving no voids in joints etc... hope this helps ....dave..