Ok. Let me see if I can explain this a bit better than usher did. Let’s use your minimarth/baby caeda example.
Let’s say that at some point on that banner you were at 3.5% pity rate and got a summon circle with 4 red stones and one blue stone. You know know a fact that the one blue stone isn’t y!Caeda so you avoid. But any of those 4 red stones could be y!caeda so you pull all of them. On the 4th stone you get a 5* minimarth and your pity rate is reset to 3%.
Now let’s say that minimarth was blue instead of red. What usher has been saying is that y!caeda still would not have been in our hypothetical summon circle. What would have happened instead is that you would have gotten 3 red stones and 2 blue stones.
What you are noticing(and is requently ignored when discussing this topic) is that y!Marth actually did male it harder for you to get y!caeda. Not because he made the rng any different, but because he was resetting your pity rate every time you summoned him, and you had no way to avoid summoning him because he was on the red color.
In our example of 1× blue stone, 3× red stones, and a stone hiding Marth, if Marth was blue you would pull the 3 red stones, not get y!caeda, leave the circle while ignoring the 2 blue stones(thus avoiding marth), and carry your pity rate into a new circle. But since Marth is red, you can’t avoid him, and your pity rate is reset any time he appears. So to an extent he is in fact making it harder for you to summon caeda by colorsharing with her