Firstly Glulx Text Effects reads the table once at the beginning of the game, so if you want to change colours later you’ll need to manually call the code. Either the individual “set PROP for STYLE to X” or call the set text styles rule to do it all again. But secondly, you can’t change the colours of existing windows, only new ones. You’d need to close and reopen it to have the changes take effect.
Or you could use the new Glk Text Formatting extension which lets you change colours on the fly, but it is only supported by Windows Glk currently (and Gargoyle when they publish a new version).