ceramic tile question

Yes.

Partially. On larger reidential tile jobs grouting can take a day or two - obviously you're not mixing that up in one batch. You'll have to take a break at some point, so the decision is where to take that break. If you are consistent in mixing the grout by premeasuring grout and water before mixing, and don't use ridiculous amounts of water cleaning off the grout, you should have very minor variations in color. If you take a break and finish a batch of grout on one wall in a corner, you'll never notice the difference.

Darker color grouts are more subject to color variations. If I'm using a lighter grout I really don't worry too much about where I knock off for the day.

The biggest difference usually comes from using too much water during cleanup. When you strike off the excess grout with the rubber float make sure you take off as much as you can. When you're using the sponge to cleanup the tile on the first go round, make that sponge almost dry. The damp sponge advice you get is really open to interpretation and a lot of people use far too wet of a sponge as it makes cleaning off the grout much easier. Unfortunately it's not good for the grout.

It's about timing - don't let the grout sit too long before cleaning and don't start cleanup too soon, and about mixing accurately.

Buy the grout "milk" if you can't do it all at once. Don't know exactly what it is called but it is a milky color and is supposed to help with color variations.

Best way is to do it all at once completely mixing the dry grout before adding anything.

If you are talking about seperate walls or surfaces, differences will not ve very noticable but if you lay in grout on the same wall two times, you may see the difference. Usually there is no good reason not to wait to grout at the end. Leave yourself plenty of time because sponging off the grout and waiting for it to setup can be time consuming.