The most common and most likely answer to this crossword clue is the 4 plus 9 letter word RACE RELATIONS
I've seen this crossword clue in The Times
R
A
C
E
R
E
L
A
T
I
O
N
S
Buddy explains!
The straight part
We know that "How different colours mix maybe" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case how different colours mix maybe = race relations.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.