fading grout

Anyone out there able to help with this grout problem - - - porcelain tiles layed in kitchen that has both timber and screeded floor base (approx. 23 sq m) layed in a day and left to cure over night. Following morning grouted and now drying out very patchy'' black grout used flexi of course but within some of the grout joints its drying white of all colours! 4-5mm spacings used - have bin tiling for 6 years and not cum accross this before! Any ideas?

Thinset is not at consistant levels (purging between the tiles) and too much water used to clean the grout initially would be my guess.