Hi Mario, all,
With regards to Efflorescence - this is the migration of salts to the surface of the grout, which when the water evaporates leaves a whitish bloom.
Mapei Ultracolor Plus cannot itself produce this because we do not use Portland cement which can release the salts. This is not to say that this can never occur as Portland cement can be found in the adhesive or substrate but as before it cannot come from Ultracolor Plus grout.
The shade variation can be due to a polymer found at the surface which can be attributed to excess water, method or tooling of the joints. The higher the polymer content in the grout the higher the flexibility and water resistance. This can be removed by lightly rubbing with a fine sandpaper.
I hope that this helps! If you have any other queries, please do call Mapei's Technical Service Deparment 0121 5086970 (option 2), open 7am - 7pm week days
Kind regards,
Mapei UK Marketing