Trimming on parasol in whiter than white colour (6)

I believe the answer is:

purple

I'm a little stuck... Click here to teach me more about this clue!

'colour' is the definition.
(I know that purple is or was a colour)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for purple that I've seen before include "Colour between 14dn and blue" , "Red-blue mixture" , "Red and blue mix" , "Mixture of red and blue" , "Red and blue mixed" .)

I've seen this clue in The Guardian.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.