I know this is quite late in posting, but I figured I'd leave my input for future readers that stumble across this article.
A lot of you speak of whitish coloring on your grouts, all of varying colors.
This is not caused by a defect, it is caused by standing water. Please read this, hopefully it will save you all some trouble.
The first wash is what we usually call the "forming" wash. You wring out a sponge until it is free of most water, and then do circular motions over the grouts and the tile, "scraping" off the excess residue on the tile and at the same time forming the grouts to a consistent shape.
After you've done this, you may want to let it sit a little while (10-20 minutes) to give a chance to the haze that is now all over the tile to dry out a little bit.
Now here comes to point where there is varying opinions. Some installers do multiple washes after this, but some only do one "wipe" with a heavily wet sponge. What we recommend is to WIPE the tile in one direction with a DAMP sponge (wring it until it is almost dry), flip the sponge and use the other side, then rinse and repeat. The result is that film or "haze" of grout getting wiped up by the sponge. If your sponge is too wet, it will leave water sitting in the grout lines. This is what you want to avoid. Even if there is no visible water sitting on the grout, if more moisture is on part of the grout than another, it will discolor. What you may want to do is with a rag or a barely wet sponge, lightly run along each grout after your final wash, wiping away that excess water. Attention to detail will get you a long way.
The important thing to remember is CONSISTENCY. You should always follow the grout manufacturer's instructions (mixing powder together, using consistent amounts of water). We like Mapei and Profix among others, any notable company's product should work fine when done right. Do consistent motions with consistent pressure, and take care to study closely what you are doing. Watch the grout lines and how they behave, you'll pick it up quickly.
TIPS: If you are dragging residue from the tile or from grouts onto other grouts, you are setting yourself up a headache. This will cause excess water on those areas and they will discolor.
On the second rinse (after you've formed the grouts) you want to wipe OFF, not wipe around. Rinse OFTEN. One pass per side of the sponge, then rinse. Wiping with a dirty sponge will only spread grout and water around.
It is important that after each rinse of the sponge, you wring it thoroughly. Trying folding it in half twice (or once, if you cannot manage) and twist it tightly.