The most common and most likely answer to this crossword clue is the 13 plus 4 letter word FORGET-ME-NOT BLUE
I've seen this crossword clue in The Times
F
O
R
G
E
T
-
M
E
-
N
O
T
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 = forget-me-not 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.