Vibrant, as crayon colors (4)
I believe the answer is:
neon
'vibrant as crayon colors' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for neon that I've seen before include "Brightly coloured advertising sign" , "Could it brighten proceedings" , "Noble gas used in illuminated signs" , "Gas used in fluorescent lighting" , "Sign gas" .)