One brand colour, a calm neutral, and contrast you measured

Choose one accent and let neutrals carry the rest. Add semantic colours (success, warning, error) only where they mean something. A spectrum or gradient is an accent, used in a logo, an underline or one hero, never behind body copy.

Check contrast, do not eyeball it: 4.5:1 for body text, 3:1 for large text and UI borders. Test the dark and light versions separately; a colour that works on ink often fails on paper.