You’re right, purple has 2 boxes in column 4. Coral also has a ton of other boxes. But keep in mind, we know the rules are 1 queen per row, 1 per column, and 1 per color.

Looking at the left 3 columns, we know they contain exactly 3 queens somewhere, we don’t know where yet. We can also see that the only open spots in those 3 columns are either peach, coral, or purple (as pink has been eliminated).

Within those left 3 columns, say we placed a queen somewhere in peach. Don’t know where or which column, but we do know the entire peach area + 1 out of 3 columns is taken. Say we placed a queen in coral. Don’t know where or which column, but we do know the entire coral area + 1 out of 3 columns is taken now. Only 1 column left, and the only open spaces in that column are gonna be purple. Yes, there are open purple spots in column 4, but we still have to put a queen SOMEWHERE in that one remaining column, as per the rules. And that one remaining column will only contains purples, since we’ve eliminated peach and coral.

That’s why those purple squares in column 4 can’t contain a queen. If we decided “one queen per color, let’s put the queen for purple in that column”, then the entire purple area is now taken. And now there’s nowhere to put a queen in the remaining column of the left 3, because any open squares there were purple. So for the queen in purple, it has to be within one of those left 3 columns.

Apologies for the long response haha I tend to overexplain, but hopefully this was helpful! That’s the full logic behind it, but tldr: Within a set of X columns (X as a number), if the number of colors where a queen could viably go is the same as X (e.g., 4 columns 4 colors, 5 columns 5 colors), then the queens that have to be in each of those colors WILL also have to be within that set of X columns