A sequence like 123,456,789 in an English language variant is read as “123 millions 456 thousands 789 units” because the comma is a thousand separator.
When you paste such a sequence into a Calc cell, a parser translates this text into a number, ignoring any comma, even if not in a thousand position. This explains why the RGB triplet ends up as a single number.
For the sequence to be considered as text, the comma must not have this separator culture-specific role. The best way to do it is to switch to another locale where the comma has no specific role (neither as a thousand separator nor as a decimal “point”).
If you paste in a single cell, the best way is to change the language attribute of the cell Format>Cells. Unfortunately, there is no [None] choice like in Writer. I suggest to choose Chinese to be sure. You can keep this attribute in case you want to modify the content, it does not hurt as long as you don’t enter Chinese data. You can also revert to your default locale once entered.
As you seem to paste a whole table, the position of the “offending” cells is not predictable. Then temporarily switch to another locale. @PKG suggested German but the comma is the decimal separator. I am not sure that this will not interfere with the paste operation (e.g. in case R or B value is missing as in 123,456). Prefer a more alien locale like Chinese. It does not really matter because you will restore your locale after the paste operation.
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