in a flash vs outright

in a flash is not an adjective while outright is an adjective.

Word	Noun	Adjective	Verb	Adverb
in a flash	No	No	No	Yes
outright	No	Yes	No	Yes

As adverbs, in a flash and outright are synonyms defined as:

Other synonyms of in a flash include instantaneously, instantly.

in a flash (adjective)	outright (adjective)
	without reservation or exception

in a flash (adverb)	outright (adverb)
without any delay	without reservation or concealment
	without restrictions or stipulations or further payments
	without any delay
	thoroughgoing