What's the difference between the red and grey abrasives when grinding concrete?

When using the RG 150 with either red or grey grinding wheels , the quality of the finish would be similar.
Using either wheel the quality of the finsish would be comparable to an 80 grit sandpaper finish. 

The main diffrence between the two wheels is the bond that holds the diamond abrasive in place. The red wheel has a harder bond than than the grey wheel has.

This is because the grey wheel is designed to be used on harder surfaces where the abrasive will wera away faster. The red wheel is designed for softer materials where the diamond abrasive will wear as fast.