The most common and most likely answer to this crossword clue is the 8 letter word TIE-DYED
I've seen this crossword clue in The Times
T
I
E
-
D
Y
E
D
Buddy explains!
The straight part
We know that "coloured a certain way" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case coloured a certain way = tie-dyed.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does TIE-DYED mean?
TIE-DYED verb- To tie strings around (fabric or clothing) and then dye it, in such a manner that the tied parts do not get colored.