The current technical features for ceramic tiles can be found in the EN ISO 10545 norm.
All tiles show certain aspects of imperfections because they are still a natural product. Research for deficiencies have to be executed according to the EN ISO 10545 norm.
This norm specifies that tiles must be looked at in a light intensity of no more than 300lux at a right angle and distance of 1m of the surface. A delivery of first choice tiles cannot exceed the 5% of tile with flaws.
There can be a difference in colour between the tile seen in a showroom and the delivered tile as this is still a natural product.