One of the most common complaints that TSD receives is regarding discoloured and patchy grout (ignoring efflorescence for this discussion). The response of the complainant is nearly always that our grout is faulty, but this simplistic assertion ignores the fact that we have a Quality Control regime which is intended to pick up this type of issue. When we receive the dry grout powder to test, we always find that it mixes normally and dries to a consistent colour. Therefore with the exception of a few unusual circumstances, the problem is related to something which has happened on site.
ARDEX created a technical bulletin on this topic (TB130 Sources of Grout Discolouration Real and Perceived) which gives a list of things that can occur with grout, but this blog post is about a specific and easy to spot problem.
When tiles are laid, part of the process is to clean the excess adhesive out of the grout lines; when the tiles are properly bedded, the adhesive squeezes between the tiles and into the grout lines. Part of the laying process is to remove this excess adhesive, preferably whilst it is still soft. However, it is not unusual for this not to happen, and then the next day, the grout is placed into the gaps and finished.
Another effect which can occur, is that because the adhesives are supposed to be applied with a notch trowel, but the notch lines are rarely completely collapsed to make a continuous adhesive bed, you have an alternating pattern of adhesive and voids under the tile. Heavier beds applied with larger notch trowels make both situations occur more easily.
What happens next is shown in the photo on the right of a recent complaint, and the explanatory schematic diagram below. As the grout dries at varying thicknesses in the grout lines, the chemical reactions for cement occur differently, in part because thinness vs. thickness changes the way the water stays in the grout to react the cement, but also due to the differing properties of the adhesive vs. any voids and exposed substrate. Where the grout is thinner it dries lighter, and where thicker dries darker. This pattern is then reflected in the grout and you end up with a set of zebra stripes in the grout lines. The intensity of this colouration pattern varies, but is pretty easy to spot with darker grouts.
Therefore, when installing grout, keep the grout levels consistent and rake out old grouts and excessive adhesive fully to ensure a constant thickness. Tile spacers must also be removed and grout not put over these plastic sections to avoid the issue of patchy grout colours.