Just my opinion.
High is an adjective and
light is a noun. That is, the brightness (noun) of the affected text is intensified, or heightened. The
light (noun) is
higher (adjective). Women had their hair "highlighted" in the 50's and 60's, which lightened only certain spots, giving (I suppose) the impression of lights shining on the high spots.
The term has been used for as long as I can remember in connection with speeches, arguments, or essays. The noun would refer to the salient points (the highlights of the argument) and the verb would refer to the process of calling attention to certain points (be sure to highlight this fact).
Highlighter as a tool for calling visual attention to parts in the text, is more recent, I think. The tool is a noun and the process is a verb (
highlighter, to highlight.) I don't often hear the resulting "highlight" referred to, but it would be a noun. You may say the text is (or has been)
highlighted.This probably didn't answer your question. Are you asking if both
high and
light may be taken as adjectives within the compound word *** ? That is, the highlighted portion is
light, or
lighter, when compared to the rest of the text. I think the answer to this is "No." Personally, I think of a highlighter as a device, or process, which imparts light to the selected text, therefore a noun
*** "portmanteau" is a new one on me. Is it French?
- A.