Yes, you are right, there will be a conflict with the numbers before the code ...
Then it can only be a new code for the colored underline. And the key combination can be, for example, this: "CTRL + Y + 04".

And also the main thing is not to forget add delete of this code to identifier $strip() and if the line already contains the usual underscore code, then it must be ignored so that the colored underscore takes precedence.

Then the following sequence may appear in the script, where $chr(??) is the code number we do not know yet:
Code
testing $+($chr(3),15,$chr(??),04,colored underline,$chr(??),$chr(3)) text


How it might look like:
[Linked Image from i.ibb.co]