How to change colour state of a portion of text in a text boxt

I have a block of text in a text box. Several of the words in the text box are key terms that I'd like the learner to read more about.

My idea is to change the colour of these particular words in the text box to red, and bold the words. Then setup triggers for each of these words, so when the learner clicks on one, a layer appears with an explanation of the term.

Then, once the learner has read the definition layer and closed the layer, the word that was coloured red reverts to black, so the learner knows they have already read the definition for that word.

I can't seem to set this up the way I mentioned above. What I have found is I can create a trigger change the colour state of all the text in a text box. But I can't seem to change only certain words in a text box.

The only other workaround I have thought of for this so far, is to setup red bordered hotspots around the key terms, and set the 'visited' states to no fill, no outline, so they disappear after the learner has read the definition for the word. This is a decent workaround, but I find the red bordered hotspots disruptive.

Can anyone suggest an answer, or another alternative workaround? Thanks.