Colour
The general colours of a UI should set or convey the branding of the company.
Colour Theory
Colours have numerous meanings and they can invoke different emotions.
Warm
Cool
The 60:30:10 Rule
60% Dominant colour
30% Seconday colour
10% Accent colour
Hierarchy
Elements of importance should contrast it's surroundings.
Varying shades of a colour can determine difference levels of significance.
Crucial Minor Less importantBe Inclusive.
WCAG - Web Content Accessibility Guidelines
4.5:1 Contrast Ratio
Make sure colours in the foreground have a high enough contrast against the background, for users who are visually impaired.
Care for the Colour Blind
Users may not be able to distinguish between different shades of a colour. Always check your colour palette for all types of colour blindness.