Uneven curing causes uneven color. Inconsistant mixing and installation techniques also cause uneven color. Successful installations are the result of uniform, good installation techniques. Checking for efflorescence. Efflorescence is the movement of moisture upward through portland cement beds and concrete slabs.
Efflorescence causes grout discoloration.
Make sure the tape seals all of the edges. If water condensation appears under the plastic within one or two days, efflorescence will most likely occur. To avoid grouting problems a waterproofing membrane may be necessary. Allow tile to set firmly before grouting. Grouting tile too soon, before the setting material has set-up, can lead to many problems.