Question about Kobo Libra Colour

This device has no such function. You can highlight text (and in colour), but it's manual. I think you need to look at specialist devices (which are not for the general public).

Kobo e-readers do not offer this function natively, but you could get around the limitation by preparing files with coloured syllables beforehand, or by using another more flexible reading platform.
One solution would be to create or find PDFs in which the syllables are already coloured. A Kobo e-reader can display PDF files, but the layout may not be perfect depending on the file and the size of the e-reader screen.