Its got to be a flexible grout and its uo to you what colour,beige maybe?seal the face of tile before grouting and then seal the whole floor when finished grouting and cleaning.
Is the 18mm ply an over lay or is this the substrate..?....if it's over lay then keraquick would be my choice ven tho mapei say mapeker will suffice...if it's an 18mm substrate then as matt sez keraquick mixed with latex plus..