The most common and most likely answer to this crossword clue is the 5 plus 4 letter word STEEL BLUE
I've seen this crossword clue in The Daily Telegraph
S
T
E
E
L
B
L
U
E
Buddy explains!
The straight part
We know that "tone" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case tone = steel blue.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 STEEL BLUE mean?
STEEL BLUE noun- A grayish shade of the color blue, like the metal steel
- Having or resembling this shade of blue