Does anyone know how to highlight a sample of letters in a text object with a solid colour using Inkscape? I saw a similar post with the question of how to highlight the entire text object (the post can be found here), but I only want to highlight a sample of letters instead of the entire thing. I don't want to add a rectangle behind the text using the rectangle tool.
No, it's not possible, if you mean to do a common thing in text editors – setting a background color of (individual) letters.
Letters in Inkscape are created from paths, they have no bounding box (“a background”) as in text editors, so there is nothing as a foreground / background color – only a stroke / fill color.
From this follows that you may highlight a part of your text only by selecting it and then changing its fill or stroke color: