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 ‘Font color’ toolbar.
Found it eventually.
Tools > Options > LibreOffice > Colors
(Press the ‘Add’ button!)
Thank you for posting an answer. 
Unfortunately, in LibreOffice 6.1.5.2, The “Colors” Option (as well as “Memory” and “Basic IDE Options”) are no longer available here.
But his may help: Calc: Use self-defined colors in style / add them to palette