I just grouted some new ceramic wall tiles which were installed on a combination of green board and cement board. The color is supposed to be light grey but after 48 hours 20% of the grout is still dark as the color of wet grout. The cement board or the green board alone aren't the problem as it shows on both. I let the premix glue dry for 24 hours before grouting and followed the manufacturers instructions. (power mix for 5 minuted, let stand 10, mix 2 minutes more. I contacted manufacturer and they said to wait another day. I don't think its getting any better. Has anyone had thyis problem Frank