See https://developer.mozilla.org/en-US/docs/Web/CSS/color_value for all details and test cases

• hexadecimal RRGGBB (#ff0000)
• hexadecimal RGB (#f00)
• rgb(0, 0, 255)
• rgb(0%, 50%, 100%)
• hsl(120, 100%, 75%)
• hsl(…,…,…) in all variations
  • With and without ° sign
  • Negative angles
• predefined color names
• CSS Level 4 predefined names (rebeccapurple)

optional:

• CSS Color Level 4: space-separated rgb(R G B[ A]) - only a draft
• gray(0.3)
• gray(30%)
• rad instead of degrees
• turns instead of degrees