how do i create and use custom highlight colors?
As you create a custom colour in the highlight dialog using the clour wheel, it is saved to your document. You can select any colour already used in the document by choosing the Document colours palette in the drop-down above the colour chart, shown below.
You might see other palettes in the drop-down that fulfill your requirements. You can also import other colour palettes (*.soc) by simply saving them to “Program Files\LibreOffice\share\palette\standard.soc” or whatever is the path to your installation.
There is one palette I use frequently because of its nicely stepped range of colours. I downloaded it from the OpenOffice forum, Palettes and Smooth colors in OpenOffice, it is called standard.soc.vertical_KG.zip. Cheers, Al