You may have letters, or words for that matter, that have become partially erased. If, after the erasure, you bring a tinok (child) and can still read the trace of the letters, meaning they are still there, then someone can trace over the letters and darken them to make them clearer. This is not considered writing the letter out of order, as long as the impression of the letter is readable and legible. Therefore, it is not a problem to trace over the letter again to darken it. But that is only as long as a tinok can read the letter. And the ink started to fade from on top of the letters.
But if, at the beginning of the writing, the ink was in black, it was like a dull color, like a white color, or like a reddish, and you have to really go over it again to make it darker, that is how it should look as well. This means we are only talking about a letter that was written correctly and then started to fade. However, if it was faded from the beginning, so that you have to write over it again, then it is considered not to be a letter. Therefore, it is considered shelo kesidran (out of order). But in this case, shelo kesidran is not considered shelo kesidran, because it remains kasher (kosher). Here, in the case of shelo kesidran, it remains kasher. What you see here is that its status is not changed by adding more ink. At this stage, it is still kasher; you are just adding more ink because you do not want it to erase further. So you are just preserving it, and therefore it would be okay. But if it was not even a letter to begin with because it was very faded, then of course it will be pasul (invalid) if you go over it afterward.
Now, this is only talking about a case where you have some of the black ink, at least, on the klaf (parchment). But let’s say all the ink fell off the klaf, and you have only some red from, say, the rust of the ink. You do not have any black anymore; you just have a little red impression on the klaf. If you write over that with a kulmus (quill), it is considered shelo kesidran. So you need to have a little black ink left on the klaf.
If some of the ink on the vav (the letter vav) chips, and then you have just a little reddish color in that spot, you have to show the tinok. That is a classic case where you show the tinok to see if you can read the letter. Wow, that little reddish leg at the bottom—you have to cover it. If the tinok recognizes that it is a vav from that part, that part is pasul. It is not considered part of the letter. So you have to cover that reddish part, and the tinok has to see if he can make out a vav from what is left over.
Another point is that you can have only a little bit of ink left on the klaf. You may have a little bit of red ink left on the klaf. This means that even if some ink remains, if the top layer of black ink has come off and only a little red ink remains on the klaf, now you just have the red part of it. I guess the deyo (ink) has a reddish color when it chips. Therefore, it is saying that red is not kasher at all. The deyo has to be black for Tefillin (phylacteries).
If the ink just becomes old and turns red because it has aged, it was written b'deyo (with ink). That is why it says in the din (law) that you have to write b'deyo, for the parashiyot (Torah portions) need only to be written b'deyo. The ink or writing is described as changing color, including to red or brown. So he says, “No, we always use black ink, and what normally happens to black ink? When it ages, it turns brown.” So that is not a problem; it just happens from old age.
Sometimes people want to say it is okay, but still, it is better to go over it. He says that for the Shemot (Names), it will be permissible. When you write over the Shemot, let’s say from brown to make it black, it is considered just tracing over the letters. It is not considered erasing and rewriting, so it will be okay as well.
But let’s say it does not turn brown. Let’s say the letters actually turn red. Regular deyo does not turn red. If anything, it may turn a little brown, or, I feel, a kind of pinkish-red. And then it turns red after writing. Or let’s say it changed right after you wrote it, which is not normal. That Sefer Torah has no way to fix it. Wow. That does not look like there was a problem of age. Something was wrong with the ink from the beginning, right? You must have made the wrong ingredients for such ink, because it was not written in deyo, in Shemot. Obviously, you cannot go over the Shemot, because we consider that like erasing.
So, to review quickly: when you have fading of the letters, as long as you have some deyo on the klaf, it is permissible to go over the letters, as long as the tinok can make out the letters.