Colors are 'washed out' / higher gamma in 4.22 mobile preview modes.
Confirmed in 4.23 MAIN @ CL 5755712
Launch any 4.22 project with Mobile Preview. (Or change in-editor rendering to High-end Mobile / ES2)
Result: Colors are 'washed out' / higher gamma
(Highly noticeable in black levels of attached screenshots)
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-72432 in the post.