Colomiers stand eighth in Pool Two of the 'Top 16' French Championship and Bath captain Grewcock declared: "Colomiers pose a huge threat. We are expecting a very tough game as they have proven in the French Championship that they have the ability to raise their game against the big sides."
Colomiers have beaten two of the top sides in the French Championship, including a significant victory over close rivals Toulouse - a side stacked with World Cup stars.
"They have beaten Montferrand away, which is no mean feat, and have also recorded a win over local rivals Toulouse, so they are a good side," said the England lock.
Bath go into the game on the back of a hard-fought 34-18 victory over Rotherham, but the change of focus for the Zurich Premiership leaders provides the chance to chase more silverware.
"We are enjoying a good run of form in the Zurich Premiership at the moment and want to transfer that success into this competition," added Grewcock."Having reached the final last season, we have tasted what this competition is all about and are desperate to make another final."