Using Libreoffice 5.1.2.2, it seems I am limited to a pre-defined selection of colours to apply as the default colour for a style.
How can I apply a colour that is not in this list? This is a colour I have already applied manually elsewhere in the document using the âFont colorâ toolbar.
Found it eventually.
Tools > Options > LibreOffice > Colors
(Press the âAddâ button!)
Thank you for posting an answer. 
Unfortunately, in LibreOffice 6.1.5.2, The âColorsâ Option (as well as âMemoryâ and âBasic IDE Optionsâ) are no longer available here.
But his may help: Calc: Use self-defined colors in style / add them to palette