The base color cannot change. That is determined by genetics. Typically, once they get regular the shade is usually pretty consistent. But things can happen during egg-laying so the shade can vary. Similarly with the egg shape. A hen will typically lay a certain shape, but things can happen where you get a variance.
A pullet typically starts out laying a fairly small egg but the longer she lays the larger the egg gets. You will probably notice a difference in size between these eggs and the size you will get in a month. Again, you may notice significant differences from one day to the next. But after they get over the start-up challenges these things typically become pretty consistent.
Many of us with smaller flocks can tell which hen laid which egg just by looking at the egg for some of them. But every once in a while one can have a glitch that makes that day's egg different. It is just part of dealing with living animals.